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. 1/2 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.
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