Experimenting with Frequency Alignment with HF+ DP/SDR# & Leo Bodnar Mini GPS Clk

Ian DXer
 

The purpose of this exercise was to determine what if any variation in frequency alignment occurs with the Airspy HF+ DP when used with SDR# R1732 & a Leo Bodnar Mini Precision GPS Reference Clock as a reference signal over a wide range of frequencies.

Setup was using the above with sample rate of SDR# initially set to 16kbps
Various frequencies were samples from 258MHz down to 63MHz, mostly in 10MHz steps & then from 30MHz down to 1MHz mostly in 5MHz steps to 5MHz, then 1MHz steps from 5-1MHz, further 100kHz steps from 1.9 to 1.1MHz & so on. The 9kHz step MW xband frequencies were also used. More later.

Nominal earlier calibrated frequency was 248MHz, but returning home later this drifted up to +0.2Hz.

Results:
1st column is the signal reference frequency. 2nd column is the SDR# offset freq. as displayed & measured in IF spectrum (ref. Zoom FFT)
258MHz +1.9Hz
250      +1.8
248      +0.2
240      +0.7
230      -1.2
220      -4.0
210      -1.1
200      +3.0
190      +0.2
180      -1.1
170      -3.9
160      -0.8
150      -2.2
140      +1.9
130      -2.2
120      +0.9
110      +0.3
100      +0.6
  90      +0.6
  80      +0.6
  70      +0.7
  65      -1.1
  64      +0.6
  63      +0.1
  30      -0.4
  25      +0.6
  20      +0.5
  15      +0.4
  10      +0.3
    5      +0.2
    4      +0.3
    2      +0.6
 1.9      +0.3
 1.8      +0.1
 1.7      +0.8
 1.6      +0.6
 1.5      +0.2
 1.4      +0.9
 1.3      +0.6
 1.2      +0.3
 1.1      +0.1
 1.0      +0.8

The MW xband 9kHz channels were also checked with results again for 16kbps sample rate.

1.611  +0.6Hz
1.620  +0.6Hz
1.629  +0.6Hz
1.638  +0.6
1.647  +0.6
1.656  +0.6
1.665  +0.6
1.674  +0.7
1.683  +0.7
1.692  +0.7
1.701  +0.8

Interesting when I changed the sample rate to 384kbps I achieved more accurate frequency alignment for most of the xband channels. Results below.

1.611  +0.6Hz
1.620  -0.3Hz
1.629  -0.3Hz
1.638  -0.3Hz
1.647  -0.2Hz
1.656  -0.2Hz
1.665   -0.2Hz
1.674   -0.2Hz
1.683   -0.2Hz
1.692   -0.1Hz
1.701   -0.1Hz
with this sample rate the carrier traces of my rxer now closely match the better aligned remote KIWI SDRers. Either way now knowing the measured offsets of my rxer I can compensate for future carrier offset measurements with xband mw stations.

As an exercise I tried aligning the HF+ to an mw xband channel, but this was a waste of time as almost 200ppb CLK steps were required to gain the slightest movement in frequency alignment. Hence as previously advised in this forum calibrate your SDR to the highest known precision frequency possible is the best advice to maintain best frequency calibration accuracy across the widest frequency range. (NB altering the freq calibration CLK for MW would have greatly effected frequency alignment in the upper frequencies, - don't do it!)

As previously advised in this forum don't touch the frequency calibration unless you know what you're doing. And don't touch it unless it can be improved on or you need to.

In summary; the  Leo Bodnar Mini Precision GPS Reference Clock is a great tool in both precisely aligning your SDR (if it ever needed doing) & for also checking your frequency alignment for a given frequency or range of frequencies for a given sample rate with SDR#.
Very simple to use too.
Helped me gain a better picture of my own SDR's alignment on the xband & helped me determine which of the regional remote SDR's were frequency calibrated/aligned correctly. A worthwhile exercise & investment.

NB the above are my results & may not be typical, others might gain entirely different results.

73s

Ian

Join airspy@groups.io to automatically receive all group messages.