[time-nuts] GPS down converter question
jimlux at earthlink.net
Wed Dec 2 00:29:28 EST 2015
On 12/1/15 6:41 PM, Bob Camp wrote:
> 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.
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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