Another Look at the BFO from The Progressive Communications Receiver from QST for November 1981

The Progressive Communications Receiver from November 1981 written by Wes, W7ZOI and John, K5IRK provides 2 of my all-time favorite analog superhet circuits: The BFO and IF amp/AGC system exalt fabulous design and function. Later, Wes -- along with Jeff, WA7MLH seemingly evolved this IF system into the hybrid cascode version presented in QST for December 2007. 

I've built 4 versions of the HYCAS IF circuit, but, I won't lie -- I prefer the IF system from the Progressive Receiver -- & I collected dual gate MOSFETs to build it back in the day. Fantastica.

BFO

I sought a 9 MHz Beat Frequency Oscillator to switch a a diode ring product detector in a new, popcorn 9 MHz IF superheterodyne receiver.   I built a couple standard BFO designs, but didn't feel happy.
To get a 50 ohm output impedance with low distortion -- plus an available power output close to 7 dBm takes a lot of buffer stage current,  filtration & effort; at least for me.  On the other hand, to make a low distortion BFO for 0 to -10 dBm available power output seems easy.

Then I remembered the BFO from the Progressive Receiver. Just 1 transistor gets it done. Popcorn! The output AC voltage is in the order of 2.3 volts peak to peak -- and the harmonic distortion is relatively low; especially for a bipolar transistor amplifier since an oscillator with a BJT amp tends to clip quicker (or more abruptly) than a JFET amp oscillator.

Above — My take on the Progressive Receiver BFO. Although an excellent design, I have never exactly copied any circuit in my life. e.g. 'I am an experimenter'.  I removed the DC from the secondary, bumped up the base bias resistors to reduce current loss through the base bias network to ground -- and found that a 35 to 5 turns ratio gave slightly better output amplitude + distortion performance @ 9 MHz.

The output circuit is part of the feedback loop so a delicate dance ensues when tweaking this interesting design. Further, a pi low-pass filter was added (918 nH inductor wound on a T37-6 dust powder toroid). Additionally, using math or software + measurement, a builder could adapt this pi filter to perform impedance transformation to boost output return loss.

The output was buffered/padded with a 5 dB pad to give an output available power of 6.26 dBm. I tuned the oscillator output with a 68 pF plus a 2.2 pF in parallel after removing and measuring the variable trimmer capacitor ( 70 pF ) after peaking the output in my DSO.

Above — The raw output of the BFO into a 50 Ω  input impedance DSO.  The BNC output port was connected to the DSO with a 50 Ω  coaxial cable.

Above — A bench 6 dB 50 Ω attenuator pad was placed between the BFO output port and the 50 Ω coax going to the DSO.

Above —  The output of the circuit shown in the schematic. Likely, the low pass filter is not needed, however, I demand a 50% duty cycle from my BFO circuit to switch a diode ring mixer. You might argue that the raw, padded output is good enough. This is a genius design after all.

In BFOs circuits with 2 - 3 transistors getting a clean signal output usually takes >= 10 mA between the stages -- or a 5th order low pass filter to mop up the mess at the end. This circuit gets it done with a current draw of ~ 7.7 mA and minimal filtration. I'm happy.

FILTER DESIGN using Web tools

From site data analysis -- just over 50% of my readers do NOT use the Windows OS. They use Mac, Linux, Android and a myriad of other operating systems. Many hardcore security-biased builders choose not to use Windows emulators -- and thus cannot run Windows-based software.  I recently got a question from a Norwegian reader who uses Mac -- How do I design my filters?

I've built up a collection of useful web tools that improve my design workflow. These mostly run java script in your web browser. For filter design, I recommend the LC design system of Marki microwave. Click here for the link  

For example, here is a screen shot of the filter I used in the BFO:

Above — My filter design by Marki Microwave's tool. I manipulated the cutoff frequency to get standard value caps ( 330 pF ) and used the plot ( unshown) to check that S11 at 9 MHz was OK. The Marki microwave site, tutorials and products prove excellent. 

Thanks and best!!

Above — This week snow drops bloomed in the garden. In the foreground the cat mint is budding. Spring emanates.

Popcorn Electronics Email Address Change Reminder -- Thanks!

 Greetings ###

The old email server went down due to Winter storm ~ 2 weeks ago --- and AC power was restored to the property only today. Several radio-related emails were on the incoming server -- and perhaps others may have been missed due to the server shutting down after the U.P.S. battery eventually died.

-  That's why I did not reply to your email message -

The old server is only checked once per month now and will get phased out by the Equinox

Please remove my old email address from your email contact list -- and the previously sent email address that automatically comes up in the TO: line when you compose an email.  The new email account is via GMail.

Click for our current email address   

Many thanks and best regards!

Popcorn Todd 

PNP Transistor Radios — What your father knew but didn't tell you

Emailers have encouraged me to publish my off-cuts. About 50% of my ideas fail. This is a tough hobby grasshopper. I first chose to not publish my VHF PNP experiments from 2024 because few people ever make PNP transistor circuits --  and as the kids say; "like  - who cares?".  Perhaps no one indeed.  But here goes.

I love PNP circuits and the rich history they hold as radio transitioned from tube (valves for you Brits) to solid state design.

Solid state amateur home brew is ancient craft

In the 1960s, using small signal germanium transistors, home brewers were making single side band suppressed carrier transmitters + also receivers with crystal filters, phase-shift networks, & even Weaver networks.  Nothing new under the sun?

Above — Click to see a solid state complete 40 meter band SSB transceiver using a Collins mechanical filter for IF filtration. The A01 transistor and all others were PNP. Still, they got it done without all the fancy programs and test equipment we've got today. I feel humbled by old circuits like this. These images came from a printed book called the Transistor Radio Handbook. This serves as my all-time favorite radio book to read since forever.  Someone has it online here.   I treasure my printed copy.

 

Above — Inspired by this Figure, I decide to make a low current, PNP RF preamplifier for my battery operated weather receiver at 162.55 MHz. Please read - the Philco 2N1742 datasheet - an fT of 150 MHz -- and whoa, look at the current gain. I'll use a few more modern silicon BJTs in my design lol.  

I love silicon transistors -- although I once build a custom fuzz 'stomp box' for a professional guitarist using germanium transistors when I served as his (tube) amp tech.

 

From reading the  Transistor Radio handbook, even in the 1960's, home brew players had enough common sense to stick their projects in metal boxes and understood how to bypass and decouple their circuits.  Somehow, despite easy summations like Mark Montrose's book above, some modern analog builders seemingly feel that the laws of physics don't apply to their projects. Curious.

 

Above — My base schematic. A PNP preamp with tuned input + output, no negative feedback (or neutralization) and a collector current of 2.4 mA. I tried old-school neutralization as shown in the inspiration schematic , but found that tapping the output coil as shown worked much better to stabilize the amp.

Input Tank Circuit 

Above — Sweep of circuit adapted to allow assessment & tuning of the input tank circuit.

For the collector coil, I temporarily placed a FT-23-43 ferrite with 10 turns on it in-situ.  I shunted that coil with a 56 Ω resistor to give a decent output Z for my sweep circuit. Thus, I knew that the frequency transfer response was due the input circuit. I tweaked the trimmer capacitor so it peaked at 162.55 MHz.

Q1 = MPSH10, a 650 MHz part still surprisingly available in Mouser's catalog for 2025. Even with the "temporary" 56 Ω resistor | coil, this amp provides a power gain of  ~ 4.6 dB.

The rest of the circuit

 

Above — "Rookie mistake". To show what might happen if one decides to not include the 39 ohm 'UHF snubber' resistor on the collector,  plus -- not tapping the collector coil as performed in the original schematic.

 

Above — The transfer function of the original schematic as shown with a MPSH10 BJT. Love this.

Most modern builders would never put a BJT as a low noise amp in a VHF project. Plus they would run a lot of current for a high dynamic range.

BJTS at VHF and above bands in the 1960s led to JFETS -->MOSFETS -->GaAsFETS | MESFETS --> pHEMT and so forth over time. But, today, we are making battery powered radio circuitry & using BJTs circa 1963.  Feels refreshing.

 

 Above — What happens  when we stick in a 2N3906 PNP?  Power gain = 2.4 dB

 Above — My fastest leaded PNP part, the BF509 soldered into the Q1 slot.

 

Above — It's fun to read the datasheet of older parts. I've got several PNP SMD parts with an fT up to 9 GHz. Even most of these are obsolete and were collected years ago when parts were real ( not so bootlegged ) and relatively cheaper.

PNP circuits are really not that different or difficult than NPN circuits. I encourage you to experiment with PNP transistors for fun and learning.

Best to you --  Spring is coming!

 

Op-ed. Having fun and receiver performance are not always related -- The fate of analog-biased home brew radio --

 

I'm only guessing the date, but perhaps, sometime in the 1970s -- a portion of home brew and certainly the professional amateur radio industry pursued high dynamic range as the be-all, end-all for receivers.

1. Like most – I appreciate a sublimely quiet, high dynamic range receiver; especially on contest weekends. Imagine our “ideal receiver” -- one that that easily detects 2 or more desired signals with an amplitude difference of 90 or 100 dB with ultra low distortion. Then too, our perfect receiver provides ball-busting immunity to the spurious responses produced by the nonlinear interaction of multiple strong signals coming into our antenna with no sweat. Ooh la la .

We’re talking about amateur radio contest-grade receivers. You generally have to run lots more current, high amplitude local oscillators, spend more money on parts -- and perhaps apply more complex circuits to get towards our perfect, idealized receiver if you home brew this stuff.  Will the pursuit of high dynamic range receivers help our analog-biased home brew hobby survive a little bit longer?  Perhaps for some.

In Canada, from surveys and also from viewing the sea of grey-haired club Hams milling around at the Ham Fest / radio flea markets, the largest demographic are those Hams aged 60-80. Many of these folks still like CW, although the younger Hams don’t. It seems that the youngsters prefer voice modulation modes, plus they apparently favor battery powered, mobile rigs. All these points might prove good goals for home brew designers who hope to attract younger Hams into home brew.

A significant portion of the ‘old-timers’ are inactive – some haven’t fired up their rig(s) for decades. Not even on 2 meters.

I’ve chatted with a few of these ‘nearly, or totally silent folks’ (who still identify as Hams and come to larger amateur radio gatherings for social contact, community and to reminisce ), Why are they silent? The answer seems complex. Partly, it was the Internet which arrived here locally in 1995. Then perhaps -- smart phones, changing tastes, changing family-life priorities, home & yard downsizing, health concerns, the digital electronics age, plus ‘tens of billions or so’ competing leisure activities may have contributed to these folks giving up operating their radio sets. What about home brew? -- I’ve got no idea. All of this and more?

With emails and conversations, some entry-level analog-biased home brewers feel intimated by high performance, contest grade projects, or advanced test procedures. Not everyone wants to go in whole hog and learn stuff like small signal analysis, or C programming, how to solve complex equations in Python, or to measure noise figure

Newcomers face more practical tasks like finding truly knowledgeable, trustworthy Elmers & sourcing affordable parts in their country. Basic skills like soldering, how to make circuit boards and understanding how to wind transformers are all skills they need to learn. As with every other hobby, they must navigate through abundant online misinformation, big meanies, and hucksters.

At 1 time the Ham philosophy to just make do with surplus or cheap gear served us well. Like most hobbies, this morphed into consumerism -- better, faster, newer, more, more, more. Now, in the digital age – some potential builders feel even more like a fish out of water.

 

Further, the asynchronous nature or automation of Ham radio has put off a few people....  I recall 1 recent, very snarky email in particular; “While I slept for about 8 hours, 15 stations heard my beacon. Yippee!     Boy howdy,  has Ham radio ever gotten exciting!!!”

Maybe synchronous communication - actual rag chews on home brew gear will provide us something to talk about?!

For some older builders, having fun, re-purposing and being thrifty were key reasons they liked home brew and even Ham Radio in general. How do we reignite the home brew passion in those turned dispassionate, ‘silent’ 60-80 year olds? Perhaps it's a lost cause? Further, how do we recruit a few younger Hams into building some old-style analog-biased radio gear? What about the recruitment of bored baby boomers into amateur radio in general?

For ‘silent’ older Hams, I conjecture that complex rigs, or projects that require them to learn to compile code into a hex file and/or write some lines of code won’t likely help recruit them back into home brew radio.

Personally knowing about presbyopia first hand, should; or how will they learn how to build with SMD parts as certain leaded parts become expensive, hard to find, or bootleg?

These are topics perhaps the home brew community might think about.

On the other hand – cheap, powerful test equipment lies abundant. For example, low cost, hobby-grade vector network analyzers. Test equipment wise – it’s never been a better time to do home brew radio.

Perhaps simplicity, fun and thrift for newcomers, or re-comers will once again serve analog-biased home brew for a few more years?

With a simple, but good receivers, third order intercept point products normally lie below the receiver noise floor during casual listening. If the front end gets overloaded and pushes the receiver into non-linear operation, perhaps they might switch on an attenuator, or turn a potentiometer to hopefully re-establish linear function – or just have to cope. It’s not the end of the world.   Having fun and receiver performance are not always related.

• Am I active on the amateur bands??

• Am I having fun ??

  Might be the best 2 metrics to embrace. Who knows? Perhaps, I’ve got it wrong and demographics and technological advancements totally determines the fate of analog-biased home brew radio? Perhaps like the horse and buggy, amateur radio analog-biased home brew’s fate is sealed.

   

• Simple, but good gear

• Emphasize and teach quality basic measurement techniques

• Thrift

• Have fun

• Learn at your own pace

• Communicate

These 6 above bold themes will inform Popcorn Electronics video content going forward.

We’ve got a lot of newly or ‘newish’ retired boomers with time on their hands. Some of them might enjoy making some home brew analog radio gear. Kits are also an option.

Best!

My 3.5X MHz Voltage Tuned VFO -- Have I gone crazy?

 

 

My Voltage Tuned VFO - Have I gone crazy?

I built a 3.5X MHz scratch voltage controlled VFO for home brew radios. No PLL circuit -- a free running variable frequency oscillator. Have I gone mad?  Perhaps. Complete design + analysis, plus a short assessment of frequency stability due to temperature changes at 2 frequency bands.

* RESOURCES *

Brad's VFO Story  How to make a temperature stable VFO
https://youtu.be/kcc8V5IMmlw

SolderSmoke Daily News -- Ham Radio Blog
https://soldersmoke.blogspot.com/ 

Iulian, YO3DAC - VA3IUL Website

https://www.qsl.net/va3iul/

Transistor Radio Series -- KO4BB Frequency Doubler -- 3.5X to 7.X MHz

 Greetings ###

I continued to seek a frequency doubler - and started experimenting with emitter-coupled pairs to make the full-wave rectifier. After several promising circuits, I discovered this circuit from Didier, KO4BB.  Scroll down to find Low phase noise common base frequency doubler

I encourage builders to check out his entire web site ko4bb.com

My work is pushing me towards simple, but good, battery powered circuits -- this implies keeping current low to conserve battery power.  This low phase noise doubler draws 3.4 mA quiescent - and delivers 6 - 6.4 dBm output power at 7 MHz from an input power amplitude of 10 dBm at 3.5 MHz. Fantastica.

Above — The doubler designed by Didier, K04BB. I used 2N4401 BJTs that came from the same reel, but did not match them. The 200 Ω emitter degeneration resistors were 1% tolerance parts.

Q1 and Q2 provide a series pass voltage regulator with Q2 as the feedback amplifier. The  Q2 BJT also serves as the voltage reference -- and provides temperature compensation. Ideal for a field mobile radio where temperature changes happen. The bias to the emitter- coupled pair is only 0.85 VDC.  I added a 100 µF electrolytic capacitor to the base of Q1 to boost DC filtration.

I designed my own output network, and because the emitter-coupled BJT pair provides such a high voltage amplitude, I eliminated an active buffer stage. This in turn, helps keep current draw low. A simple 32 turn to 5 turn ferrite transformer goes into a low pass filter plus a final -4 dB 50 Ω pad.

The proof of how accurately the input signal gets full-wave rectified is demonstrated by the amplitude of the 3.5 MHz input signal at the output.

 

Above — The spectrum analyzer transfer function.  I placed a 10 dBm signal on the input and the full-wave rectifier suppressed this by ~ 53.5 dB indicating good balance. The output = 6.32 dBm, perfect to drive a Level 7 diode ring mixer.

Above — Before I tested the output signal in a spectrum analyzer, I connected it to a 50 Ω input 'scope channel. Even here, you may see this doubler functions fairly well. This is the best 3.5 MHz doubler I've built. Big thanks to Didier, K04BB for sharing his design -- it's a keeper.

Best!

FET notes [ J310 ] — 3.5 MHz VFO Frequency Doubler

 Greetings ###

In the past few years, 2 readers emailed asking for a 3.5 MHz doubler for their VFOs. I encouraged these readers to look at EMRFD, radio handbooks & other resources. Today, I built 1 -- and tried to solve some gain concerns with passive doubler networks described by the 2 readers.

The bog standard passive doubler "a.k.a. full wave rectifier" features a trifilar wound transformer, 2 diodes plus an RFC (plus, or minus an additional resistor + bypass cap as shown in EMRFD).  ** see last image **

You'll often get insertion loss of ~8-9 dB. If you connect 10 dBm available power to the input, the 10 dBm @ 3.5 MHz becomes ~2 dBm @ 7 MHz. This is par for the golf course. C'est normal.  

However, when you try and match the doubler output Z to 50 Ω via transformer ratios +/- a parallel tank resistor, you might even drop the signal another 4-5 dB depending on your method. Losses in #43 ferrite wide band transformers might also factor.  Further, after that, perhaps, comes potential filter losses. This is what the 2 readers told me -- and I validated their concerns on my bench.

Above — Today's experimental 3.5 MHz doubler. 

I reckon that like me, many people seeking a 3.5 MHz doubler want to drive a 7.X MHz Direct Conversion receiver LO port with a ~3.5X MHz VFO. The doubler goes in the same container as the mixer/product detector and functions to help reduce 7 MHz leakage getting into the mixer's RF port, the antenna and so forth. 

Leaked 7 MHz LO signal can react with the local environment -- for example suffer phase shifting, 60 cycle modulation and other sequalae -- and then come back into the product detector via the antenna. Among a myriad of other problems, LO leakage may lead to DC on the mixer ports upsetting mixer balance by changing mixer diode bias. The whole goal is to try & contain the 7 MHz LO signal to the mixer LO port &  within the mixer chassis. 

This also involves carefully filtering the DC, however, that is a topic for another day.

To that end -- the first RF amplifier is our familiar common gate job with at least 30 dB of reverse isolation (Measured 33.7 dB). This keeps helps the 7 MHz from leaking down the chassis LO input coaxial connector and coax. I bench matched the input port to 3.5 MHz with a return loss bridge -- and achieved a return loss of 25 dB with the pi match shown.

A standard doubler circuit goes on the FET drain -- I put in some Schottky rectifier diodes that were handy. I didn't match them, as previously when I dynamically matched pairs of these diodes in a home brew single balanced mixer ( 2 diode circuit ) going into a spectrum analyzer, this old bin of diodes seemed remarkably close in characteristics.

I viewed the suppression of the even harmonic LO products
e.g.  --  2LO ± RF, 4LO ± RF ....  as my measure of balance of the diode pair.

I prefer a tuned tank on the diode output -- standard stuff. If you match the tank to 50 Ω with the turns ratio of a secondary winding, typically, a parallel resistor across the primary is required. Doing this, I've measured an output return loss of 18-20 dB at HF in the past. However, the losses make you sad. I want a minimal output amplitude of hopefully 4-5 dBm available power to switch my mixer diodes.

To avoid losing gain, I AC coupled the hot end of the tank to a JFET source follower standing 13 mA drain current to keep the lower , or negative 270 degree AC signal peak from bottoming out when driven with 10 dBm input power.

Above — DSO capture. The raw doubler output before the low pass filter was installed.

Above — DSO capture. The doubler output with the low pass filter.  Driven with 1 V pk-pk or 4 dBm.

Above — DSO capture. Now at 10 dBm available input power. The output calculates to 5.1 dBm. It's OK.  You can still see lots of distortion in the signal.

Above — Spectrum Analyzer capture.  Sadly, when I measured this, I failed to tune the tank after I added the low pass filter to the breadboard. I must have bumped the tank or something. So, the earlier shown DS0 captures feature the correctly peaked RF tank, but this does capture not. Face palm.

Even still, the 3.5X MHz signal is down ~44 .5 dB relative to the 7.0X MHz signal peak.  Not too bad.
I might tweak this circuit some more. I though about reducing the losses incurred with the #43 material trifilar transformer. Because this circuit works at 3.5 MHz, the transformer does need a fair amount of inductive reactance and a #61 material transformer would require more windings... 

I do not want the 3.5 MHz LO to go more than 10 dBm output power to help reduce LO pollution into my radio receiver system that also includes the antenna.   Thank you and Best!

 

** see last image ** EMRFD.  Best book ever written on many RF topics -- including Direct Conversion receivers.

FET notes [ J310 ] - common gate amplifier termination insensitivity

Greetings ###  I'm slowly working on circuits for my next video. I sought a common gate amp with a well-defined, wide-band, output impedance to go on the RF port of a diode ring mixer. I also matched the input to 50 Ω for the entire 40M amateur radio band.

I'll show more hand drawn schematics going forward. I only post a fraction of my experiments -- partly, because I'm lazy; but it takes considerable time to draw schematics with software. Thus, I often don't bother sharing much content nor post that much.  My writing is Ugly style.

 

Above — Common gate amplifier for 7 MHz. This features a TO-92 J310 with a drain current of 12 mA. When raw bench experimenting, I use size 50 toroids -- later if it turns out I only need 15-18 turns of wire, then I may down size to a # 37 toroid etc..

Above — Amplifier measures. Gain = 6.43 dB. From my experience, you'll only get the often mentioned ~10 dB gain when you tune the drain -- plus run an air wound, or a dust-powder toroid like the T50-6 on the output tuned circuit. The #43 ferrite exacts losses, but works OK for wide band transformers -- and in combination with the 2K0 resistor -plus- a drain ferrite bead, or 22-51 Ω drain resistor, behaves itself. (Does not usually oscillate). 

Typically, we keep the FET's gate lead length to the ground plane short [ low inductance] to boost stability as the J310 is a VHF-UHF part.

The common gate or common base topology output impedance proves difficult to well match to 50 Ω using just transformer turns ratios. The usual method is to place a resistor in parallel with the primary coil. This works well & I chose 2K0 Ω for that resistor. 

The turns ratio reflects the Z transform from 2K to 50 Ω. I measured the output return loss from 2 to 13 MHz . The output return loss stayed above 20 dB from 2.1 to 12.8 MHz.

Then I sweep tested from 7.0 to 7.4 MHz. The output port return loss measured 22.8 dB across the whole 40M band. I'm happy. A decent match for my product detector RF port.

To measure return loss on the input port, we place a 50 Ω terminator on the output port. Then to measure the output port return loss, we switch ports-- and place the terminator on the input port.

Above — A wide band BNC 50 Ω terminator. I've got these and SMA versions that go from DC to 18 GHz. You may also make your own with a BNC/SMA connector and resistor(s). We use them to measure the directivity of our return loss bridges -- and calibrate stuff like VNAs.

Note in the data, I performed 1 more measure on the input. I checked the input return loss with no 50 Ω terminator on the output port - open circuit. The return loss across the 40M band = 18.6 dB with an open circuit output. Amazing. Recall, that many RF amplifiers are termination sensitive.

Termination sensitive means that the impedance or resistance seen at the output port may affect the input impedance. In this case, the amplifier input return loss remained ~OK despite the output port going open circuit. 

Further, I measured the reverse isolation of this amp at 32.5 dB. I believe strong reverse isolation is the primary reason that the amp seemed fairly termination insensitive.

This amp seems special -- and it is. The common base (BJT), common gate (FET), and a cascode of 2 FETS, BJTS or a FET + a BJT offer better reverse isolation than standard common emitter/ common source amps.

Above — A sweep in my tracking generator-spectrum analyzer. This device says the gain = 6.65 dB  @7 MHz . The low Q pi network offers some low pass filtration - bonus points.

This RF preamp will go in a HF direct conversion receiver. In the past 25 + years when I've done this -- on 3 occasions, total strangers took the time to email me & wrote that I'm trashing my receiver dynamic range -- that I don't need it -- and to get it out of there! 

Oh no -- its the RF preamp police lol!

I'll say it here -- it's OK to put in a RF preamp in an HF receiver if you want to. Attenuate or boost the signal before the product detector/mixer if you like. You do you. There may be a good case for RF preamps at HF in some circumstances.

Throughout solid state amateur radio history, some rather clever builders placed common gate (or common base)  preamps in front of direct conversion or regenerative receivers to reduce the amount of LO signal at the RF port going out into the antenna, doing bad stuff and ending up back in the mixer. (I'll be covering this in the video).

Perhaps, the listener receives with a subpar antenna? For example, with a small loop antenna -- and needs a little bit of RF gain?  Further, not everyone is an amateur radio contest listener. Many just like to tinker with simple circuits and enjoy making signals bigger.

Above — Another case? The transmit output amp of my old 7 MHz Funster trans-receiver. The receiver is connected to the antenna via a low Q series-tuned network. During transmit and a for a few milliseconds after, the pin diode pair shunts the PA RF output to ground.

On receive, the signal from the antenna passes through the low pass filter, then through a XL = XC = ~400 Ω series tuned network ( 56 pF + 9.1 µH ) to the receiver input. The series resonant circuit + low pass filter exhibits 8.8 dB of losses. A 6-8 dB gain common gate amp might help builders who choose the series connected transmit/receive scheme shown above.

An RF preamp is probably not required in most cases for HF reception. However, there are always exceptions.  A strategically placed amplifier with high reverse isolation might prove helpful and boost your receiver function and enjoyment.  Ultimately, you're in charge. Do whatever you like.

Best to you!

Part Obituaries continued : R.I.P -- dear J310

 Greetings ###

I'm working on a common gate RF amp using the ubiquitous RF home brewer's JFET, the J310. I happened to go on DigiKey and Mouser. Here's what was written for the part I was looking at on DigiKey:

The 10-volt or the higher voltage SMD versions are no longer made!  The TO-92 versions said goodbye a few year ago. I checked some of the substitute parts --- many of them are also no longer made and are just stock.

Yes, people are selling real & bootleg TO-92 versions on EBay etc. -- and also small-scale sellers -- or sellers abroad sell TO-92 J310s.  However, for the average person around the globe, getting these now expensive parts -- and paying shipping costs seems quite a bother. I now have to think carefully when I make a schematic and suggest people use a TO-92 J310 in the 'FET slot' like I casually did 20 years ago.

Fortunately, I saw this coming and actively searched for a "lifetime supply" of TO-92 JFETs around 1998-2002. I purchased hundreds of FETs back in that time slot.

Above — Some of my collection. The leftmost bin also houses 2SC3355s, a 1 GHz NPN part with a low noise figure. I've got BF244 (a 700 MHz nJFET part), 2N5484, MPF102, J310 and a few others in TO-92 for RF. I've also got popular switching JFETs like the J111, J112, J113, & J176 ( p-channel JFET) in my audio parts bins. All these parts are originals and many were in dusty factory packages.

Above — My TO-92 J310 bin is now gold? I bought in minimum quantities of 500 back in the day. Got them for 10-15 cents each when buying in bulk.

 
Above — Top view -- TO-92 J310 bin. About 3/4 of them are Fairchild Semiconductor versions made in 1997. VDS max = 25 volt versions. I also gave away hundreds of J310s plus other parts to some readers of the original QRP HomeBuilder web site for free between 2003 and 2016.  Before shipping got crazy expensive. Also, now to send parts, you have to fill out extensive paperwork.

Above — I have 2 original U310 JFETs. If the J310 is gold, then these are 'platinum'. 

I also collected some ultra low noise through-hole  JFETs for HiFi audio and noise measurement circuitry. These particular FETs now cost a small fortune -- and most affordable versions are no doubt - bootleg.

If you can find a good deal on real TO-92 J310s,  now is the time to grab them. I'm going to probably lean towards using surface mount FETs in future projects for Popcorn. At least people can still get these for now. It's sad to think that such a integral part of home brew electronics is going extinct. 

Analog epilogue -->  Gone extinct -  gone digital,  bye,  I'm gone.  Goodbye old friend.

Addendum:

I've been thinking about mixers a lot. I home brewed 7 different styles to learn more about them.  Just musings. Not to build. All major content for Popcorn will be presented in videos. The audience is not exclusive to Ham or SWL radio -- it's for all people who like straight-forward, accessible, analog electronics. I've got some great ideas to hopefully share. Thanks.

How People Make Circuits without Printed Circuit Boards - Video

 
Addendum added to the text January 13, 2025 - scroll to see -

Resource Files

 

 

 

Addendum January 13, 2025

LOL -- a reader emailed and wants more signal power -- and asked whether they could just swap in a pair of medium power BJTs like the BD139-140 to get more output power. Sadly no. I have a higher power version of this amp in a receiver called Regen 5 plus one other on this Blog. However, there is another way to get more power without greatly increasing the parts count:

Above — My new Popcorn PA version that goes to 700 mW clean signal power without adding a ton more parts. The finals = a TIP 122 &127 pair. 2 more diodes get added to the bias stack to forward bias the 2 pairs of emitter followers. To avoid a big voltage drop, I moved the 22 Ω resistor outside the TIP power followers on the DC positive rail since these BJTs draw much more current than the basic version of this audio PA. Further, a 470 µF AF bypass capacitor gets added to filter the output pair on the positive DC rail.

The TIP 122-127 are low voltage complementary power Darlington transistors in a standard TO-220 package giving a current gain of ~ 1000. They only cost ~ 60 cents per transistor even in Canada with our nearly worthless Canadian dollar.  A BD139-140 transistor usually cost 25-30 cents more.

Above — Schematic and pin out of the TIP 122-127.  

Above —DSO screen capture at about the maximum clean signal power ~ 700 mW.

Above — FFT screen capture with the AF amp driven to 700mW. The harmonic distortion is not as low as the original audio PA shown in the video. However, this is the compromise of using the TIP Darlington transistors + easy-peasy diode biasing scheme with 4 diodes. The better way to bias the power followers is via a transistor like I did in the audio PA Regen 5 back in 2015.  Regardless, the TIP pair are a compromise compared to 2 separate BJTs pairs with optimum circuit design.

Still, to, the harmonic + crossover distortion will still be on par; or better than most of the audio amps you see in many lower power consumer electronics -- or a typical home brew Ham /SWL radio receiver.

Thanks for the email. If you have a question,or request,  please email. 

My readers are the best!   Popcorn Todd