Band-I Link Receiver Circuit

[reverend]

Here's a circuit for a really simple Band-I receiver without too many parts. It's not crystalled or synthesised, however it uses the AFC from the CA3089 to keep the receiver locked on frequency. This works pretty well as long as the oscillator doesn't drift too far and works even better if the band is relatively quiet so that it doesn't drift onto someone else's dial.

It has a carrier sense relay built in but this can be omitted if you don't need it. The coil that is connected to the oscillator could be replaced by a crystal with very few other changes (and the AFC wouldn't then be needed!)

Rev-Band-I-Link-RX.jpg

Rev

[radium98]

our freind in forum Sinus also provide a good schematic too

[reverend]

Here's a 2-sided PCB layout (excluding the top layer ground plane which isn't shown otherwise it would hide the traces on the bottom of the board. If anyone's interested, I could post the Gerber file?

pcb-layout-rev-band-i-link.jpg

Rev

[Rfdiva]

Go ahead thank you .if you have it in .lay ? If not the gerber will do thanks.

[reverend]

Here you go. Here are the Gerber files for the PCB.

Rev2.02b Band-I RX.zip

Incidentally, the reason that the majority of components are surface mount but some are not, is that the ones which are not SMD need to be changed if:

• there is a significant change required in the operating frequency (i.e. those around the input and the VCO), or

• there is a significant change in the wanted deviation of the received signal. The RX is set for 75 kHz deviation with ~250 kHz channel bandwidth but this can be reduced to 50 kHz channel bandwidth and lower deviation (say +/- 15 kHz) with some changes in the resistor/capacitor values around the CA3089.

Rev

[Rfdiva]

Thank you.what about a tx and or aerial .

[Albert H]

The Reverend's circuit isn't too far away from the one I used to use, though I tended to use either a crystal or a PLL for the LO frequency. I also added a preamplifier stage (just an aperiodic BFR91 amplifier) to better match the aerial to the input tuned circuit. This gave a massive improvement in both sensitivity and image rejection.

I also included a two-stage 107 MHz transistor amplifier between the mixer output coil and the input to the filter before the CA3089 - this also increased the sensitivity a lot.

It's really easy to build this kind of receiver for ~500MHz, using Lecher Bars instead of coils. I used etched lines for mine, and the receiver worked really well and was easy to align. I always matched the input to 75Ω so that I could use TV aerials and coax.

If anyone needs a link transmitter circuit for Band I, I can put something up later.

[XXL]

Yes put one up.