70MHz Receiver Front End

By | 25th March 2018

Back on to the homebrew 70MHz SSB transceiver project having been consumed with work and stuck in a “over thinking” loop re-designing the 70MHz transceiver receive front end module I have now completed a working unit – third attempt!

So, to cover off my initial attempts I was stuck with problems like little or no gain, parasitic oscillation and this forced me to re-think the PCB layout and make some design changes to better improve stability, however my second attempt had plenty of gain but I was unable to stop the 70MHz pre-amplifier from self oscillating.

I looked at an old design from a Practical Wireless article from 2006 by Tony Nailer G4CFY of Spectrum Communications in Dorset known as the Poundbury, I did not wish to simply copy Tony’s design but used it as a benchmark for my own design and it got me on the right track, the front-end comprises of a tuned 70MHz pre-amplifier and a 2 stage 70MHz band pass filter, both the input to the pre amp and the output from the band pass filter are 50 Ohms, this was achieved by using a tapped capacitor whcih you can see in the schematic below – another article to follow on the complex calculations for split/tapped capacitor matching networks, I spent a frustrating week trying to figure this out with help from various Radio internet forums and LTSpice simulation software.

The 70MHz singnal is fed in to X1 and matched to gate 2 of the BF981 dual-gate mosfet by C1/C2, D1/D2 are to protect the input to the RF pre amplifier. X4 provides connection for a 10K logarithmic potentiometer to allow for RF gain control, the range of the gain control is not great but enough for this application. The pre amplifier provides around 16dB of gain, more than enough as I found anything above 18dB just makes the circuit oscillate and noisy, the band pass filter formed by L3 and L4 is terminated into 50 Ohms allowing direct drive of the SRA-1 mixer – note the SV1 jumper connector, this allows me to bypass the 70MHz pre amp for two reasons:

  1. I can connect a HF antenna direct to X6 and listen on other bands for test purposes.
  2. If I jump pins 1 and 3 I can monitor the output of the 70MHz pre amp and band pass filter through X6 allowing to me peak the pass band and gain correctly on the spectrum analyser.

The Double balanced mixer Z1 is followed by a 50 Ohm diplexer and the 10.7MHz IF signal is amplified by a classic 2N3866 class A post-mixer amplifier and terminated into a 6dB pad, the VFO is injected at X2 with a level of +7dBM at a test frequency of 80.9MHz (This put the receiver on 70.200MHz – high-side injection using an SI5351 and MMIC buffer amplifier)

I hooked this up after initial alignment to my Icom 7300 which I had tuned to 10.7MHz to act as the IF stage, I was pleased with the overall gain and low noise level however I don’t currently have a 70MHz antenna up so I moved the VFO to 17.7MHz and bypassed the pre amp using the jumper as described earlier – the 7300 was alive with signals from 40 meters using a random length of wire connected directly into the mixer Z1.

Below are some pictures and the current revision of the schematic, next step is the IF amplifier module which will be a variation of the classic Hybrid Cascode design by W7ZOI.

70MHz front-end pcb 2