Re: [Discuss-gnuradio] IF Hardware Question

[Marius Hauki]

Hi Ian !

If your WFM keramic filter is still left in there and you intend to
use the WFM modus on the radio you should
tap the signal off using a high impedance and low capacitance
amplifier. The MMIC's from Mini Circuit is not good for this job
since they have a low input impedance around 50 ohms. If you load to 
hard, you will mis terminate the input side of your WFM filter and alter 
its characteristic in the worst case. You could maybee use an Opamp with 
large BW, like a video amp. or similar. Or, you could insert a FET 
amplifier in front with capacitive coupling just like "an active RF 
probe". Just make shure you do not have to many reactive elements in you 
r tap off chain so you get a clean phase and frequency response in the
passband. If you have problems a high isolation PI damper could
get rid of the reflections. A broad low pass filter could also be good 
after the FET or amp. to get rid of any wideband noise into your A/D.
Maybee your radio is good enough so you do not need the low pass filter.

Best regards
LA9EEA
Marius Hauki


Ian Wraith wrote:

> Hello
>      A few questions for any hardware persons on the list ..
>
>  I have extracted a 10.7 MHz IF from the input side of the WFM ceramic
> filter on my receiver (which isn't easy easy to do on an AOR3000). I really 
> need to amplify this a little before mixing it with something else to obtain 
> a lower IF. Am I correct in presuming a ceramic filter is high impedance ? If 
> so should I use a Toko IF transformer to match the signal to a simple 
> amplifier ? If that is the case can anyone suggest a suitable Toko part
> number 
> for this component ?
>
> Sorry for the all questions.
>
> Regards
>
> Ian
>
> address@hidden 
>
>
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