Monday, April 24, 2017

"Telrad" board to work


Just spent part of the weekend populating the "gig" with auxiliary circuits for the "Telrad" board testing, following previous posts.

Nothing special, a 9.000 Mhz BFO an LM386 audio amplifier and the power circuit, vfo is external. Not using the +12/0/-12 for supply, only +/- 9V from small batteries.

VFO is from a si5351 controled by the Arduino, signal level from the VFO is reduced to show around 100mV on the mixer carrier input pin (10)

BFO is from the Bitx and other designs directly connected to the board BFO input #11:

No provision with serial inductance or capacitance, at the moment, to "tune" the frequency.

Audio amplifier is a stock LM386 design and interestingly got some audio oscillation from the circuit that I will fix on latter experiments, probably with a RC low pass filter on the supply voltage. Connection was made at the audio output of the board and now is at the isolation transformer out.

Test signal is injected from my S9 signal generator to the front end and I do get audio ! Success...but only with injection bypassing the inboard front end filter and directly to pin 1 of the first MC1496 mixer.
Signal level injected from the signal generator even without proper measurement and calibration is probably around S4.
Audio is on the low side so more testing is needed.
I didn't managed to get LSB, only USB, have to check why (proably switching circuit) although if injecting 9Mhz directly on the LSB filter output I could be ear the signal with low level.

Another view of the first success:

I chosed 3.6Mhz since, apparently, that's around the front end filter center frequency...I start to suspect it's not, still need to bypass the filter to get audio out.
Probably in the future will remove the filter components from the board (last resource) and use a pre-amp also.
Injecting 9 Mhz on the front end is also audible as an audio tone, that's expected and is also a confirmation the board is working.

Will post more info as soon as more experiments are made.

Have a nice week!

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