[time-nuts] Frequency Comparator Ideas needed

Bob Camp lists at rtty.us
Thu Aug 11 21:48:40 UTC 2011


If you need to know when the shifts occur in tens of ns, that significantly
limits what you can do. Getting > 20 million readings a second is tough. If
this is an off the air signal, it's noise is probably not good enough for
you to estimate it accurately enough in that time frame. 

If this is FSK, then modulation theory gets in the way a bit. 150KHz
instantaneous shifts generate a lot of energy at the shift point. You also
need to worry about dynamic range in addition to signal to noise. 

How about a simple FM discriminator with a bandwidth around 10 MHz and a
center at 50 MHz?. Feed the output into an A/D and take it from there. It's
probably as good as anything else you could do.


-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Dan Kemppainen
Sent: Thursday, August 11, 2011 4:11 PM
To: time-nuts at febo.com
Subject: Re: [time-nuts] Frequency Comparator Ideas needed

Hi All,

I could clairify things a little. My event of interest is basically a 
fast frequency shift of a signal that drifts between 1800 and 2600Mhz. 
There is slow drift of this signal of many hundreds of Mhz, with fast 
frequency shifts of approximatley +/- 150Khz. I believe that the 
150Khz shifts are nearly instantaneous, but currently have no way to 
measure exactly how fast they occur. Slow drifts are corrected for by 
a loop in the down converter.

Currently there is a first down conversion stage to ~900Mhz. This is 
then down converted again to an arbitrary frequency band (50Mhz in 
this example, but this could be moved from ~10Mhz  to 200Mhz or greater).

Obviously it would be advantageous to keep the low frequency band as 
high as possible, at least when trying to determine when the frequency 
shifts occur with any digital detectors. Probably for any analog 
detectors also.


Obviously I'd like to get as close to the real zero crossing as 
possible, but I'm sure I don't need 0.6pS. If I could tell if the 
signal was within maybe +/- 15 or 20Khz, it may be acceptable. I could 
always double the frequency then down convert to increase the 
deviation. Basically, I have another timing device that will record 
every single event within 5nS, as long as they don't exceed ~1e6 
events per second. That's the unit I'll feed the signal into, and what 
I need to keep happy.

I did just learn about Gilbert cell mixer that works from DC to 500Mhz 
yesterday. I didn't realize they were available with that wide of a 
bandwidth. With this part as an option, it may be possible to do a 
quadrature detection around the several hundred Mhz range. That way 
subsequent low pass filters can have fairly high bandwidth. I'm sure 
the results here will probably be noisy, but maybe still acceptable???

However, I'd still like to it digitally if possible. Maybe even adding 
a second VCO with high modulation bandwidth to use a PLL with to track 
the input signal. Maybe then a phase detector comparing that PLL 
output and the reference signal and some high speed digital processing 
could prove useful. Not sure tho, still need to think about that. 
Probably would end up needing some multi Ghz flip flops to make that 

Ultimately this may not be possible digitally, but I thought if anyone 
here knew of anything it would be here...

Thanks all!

time-nuts mailing list -- time-nuts at febo.com
To unsubscribe, go to
and follow the instructions there.

More information about the time-nuts mailing list