They both appear to be discontinued. I've looked for chips on RS, Farnell, Mouser etc that would do this for me but I haven't found any - I'm most likely looking in the wrong places. Are there modern equivalents to those two chips?
Would it be possible to use the oscillator in the NE612 in a Colpitts configuration, and achieve the frequency range by using a varicap?
I can't help feeling this mixer is getting to be way too sophisticated for such a simple task!
...
I built a heterodyne bat detector about 10 years ago, and just used an 'off the shelf ' bog standard 4052 CMOS multiplexer as a switching mixer. Because the LO for a switching mixer is a square wave, only the 'signal' input - and its inverse - need to be linear. Much simpler !
(For the maths, see page 6 of this paper:
https://www.analog.com/library/analogdialogue/archives/43-09/edch 4 rf if.pdf )
The rest of the circuit (including the oscillator) used TLO72 opamps. Using an oscillator in that configuration removes the need for power supply regulation to maintain oscillator stability.
I've attached a handwritten sketch of the mixer ..(in a previous thread I spent
ages drawing out a circuit for someone with one of these computer 'schematic drawing packages', and no one even bothered to read it... so a pencil sketch it is for the present!
)....
It's just the mixer and oscillator part of the original circuit. The preamp is probably not really what you'd be needing with your MEMS mic... 10 years ago I was restricted to using an electret mic - with a less than ideal ultrasonic response - so the preamp was built to compensate for that.
In the UK (and as you mentioned receiving BBC I'm guessing you're in the UK?..) the main frequency band of interest is likely to be from 25KHz (noctules) through 50 - 55Khz (pippistrelles). Serotines and Daubentons are also covered in that range.
The Horseshoe bats are up around 120 KHz, but they are so rare I didn't bother to try and cover that range, with a less than ideal microphone!
There's a small mp3 here :
www.jp137.com/las/soppips.mp3 that I made a couple of years ago of some local soprano pippistrelles... the oscillator for this recording was set around 55KHz.
Another thing I did differently (although I did fit an LM386 at first) was to ditch the power amp. You can drive headphones directly from the output of a TLO72, when the ouptut is fed via a low(ish) value output resistor ... say 100R. (any less and the amp might oscillate, with the reactive load presented by the phones.
In practice, I fed the output from the detector directly into a cheap audio recorder (I use a Sony like this model:
https://www.amazon.co.uk/dp/B00BOK8ZB6/ref=dra_a_cs_hp_hn_it_P1700_1000?tag=dradisplay0bb-21
and plugged the headphones into that. Increases the life of my single 9v pp3 battery in the bat detector considerably....
Just a few suggestions you might like to consider for your project......