[App_rpt-users] Hamtronics receiver issue with AllStar RTCM

[Joe Leikhim]

If you are using an external reference, you cannot make up for the 2nd 
LO error if it is wildly off. There is an easy SINAD related test in 
EIA-603 that is called Offset Acceptance Bandwidth that would indicate 
if the IF were off center. Things that could go wrong apart from simple 
misalignment of the Quad Detector coil could include the LO crystal 
being way off frequency, the IF filters being wrong BW or off spec pass 
frequency.

Given the symptoms, I am putting my money on a misadjusted quad coil.

Joe

On 3/28/2016 5:06 PM, Bryan Fields wrote:
> On 3/28/16 2:51 PM, Joe Leikhim wrote:
>> 1) Make sure the receiver First Local Oscillator is adjusted correctly
>> on frequency. (Fo -10.700 MHz) Likewise the second LO should be at
>> around 10.2450 MHz. The second LO is not adjustable, so any minor error
>> would have to be made up with the TCXO U3. After this second step:
>>
>> Then :
>>
>> 2) Make sure the general alignment is set properly, especially the
>> discriminator coil T2 which responds to 10.7000 MHz from a signal
>> generator, has an S curve response.  In my experience with these sort of
>> Quadrature  Detector chips, you can get a false sense of IF centering
>> where the noise component is suppressed. You might try adjusting for
>> maximum noise first without the 10.7000 MHz signal. Be careful you don't
>> jam the coil slug.
> These are both very good points i didn't consider.  I normally don't check my
> reference oscillators in the hamtronics as they are using an external 10MHz
> source.
>
> You will want to check SINAD on it as well, if it's not making spec (and those
> hamtronics are very sensitive), something else is wrong with it.
>

-- 
Joe Leikhim


Leikhim and Associates

Communications Consultants

Oviedo, Florida

JLeikhim at Leikhim.com

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