[time-nuts] GPS down converter question

Jim Lux jimlux at earthlink.net
Wed Dec 2 00:29:28 EST 2015

On 12/1/15 6:41 PM, Bob Camp wrote:
> Hi
> So back to the question …. does a 70 (ish) MHz fancy filter really buy you
> anything ahead of the main box? If you will be multiply band limited ahead
> of the mixer (antenna and saw), the contribution of the 70 MHz filter will
> likely be minimal. Note that I’m comparing a filter that costs money to a
> L-C tank that is essentially free rather than truly no filter at all.
> ========
> One other thing to consider - Most antennas have a *lot* of gain in them.
> Back when I took the course, the standard advice was to put as little gain
> ahead of your mixer as possible. It’s all tied up in the impact gain has on
> the distortion products. That also would drive you towards a filter ahead
> of the mixer approach.
> Bob

Do you have strong interfering signals close by that might be an issue.. 
70 MHz is in the low VHF TV band.  These days, in the US, TV is all 
digital, so you don't have that big carrier to worry about, but a lot of 
GPS receivers that use 1 bit ADC are really hit hard by a narrow band 
signal that is big enough to "capture" the sampler.

JPL's receivers use a fairly broad L-band filter (covers L1, L2, and L5) 
before the LNA, then narrower filters for each subband before the 
sampler. Strong L-band signals are a real problem if they cause 
intermods in the LNA.  In space, that's not usually an issue, but in 
terrestrial testing of these receivers it is.  And in receivers designed 
for precision geodesy or timing, they typically hate to have narrow band 
filters, because of the delay through the filter.

They're direct sampling, so conversion to IF isn't an issue for JPL, but 
if you are doing a down conversion, if you have a strong L-band 
interferer that is either generating intermods, or is at the image 
frequency, you might run into an problem.


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