Worked on the calibration of the RF AM Measurement Unit.
The calibration concept is as follows:
- Generate AM modulated RF output
- Measure sideband amplitude using a network anayzer (HP4395A). This gives us the SSB carrier-sideband ratio in dBc.
- Measure the output of the RF AM measurement unit with the same RF signal
- Determine the relationship between dBc(SSB) and the output Vrms.
The AM modulated signal is produced using DS345 function generator. This FG allows us to modulate
the output by giving an external modulation signal from the rear panel. In the calibration, a 1kHz signal with
the DC offset of 3V was given as the external modulation source. The output frequency and output power of
DS345 was set to be 30.2MHz (maximum of the unit) and 14.6dBm. This actually imposed the output
power of 10.346dBm. Here is the result with the modulation amplitude varied
RF Power measured Monitor output
Modulation with HP4395A (dBm) Measure with SR785 (mVrms)
1kHz (mVpk) Carrier USB LSB MON1 MON2
0.5 9.841 -72.621 -73.325 8.832 8.800
1 9.99 -65.89 -65.975 17.59 17.52
2 9.948 -60.056 -59.747 35.26 35.07
3 9.90 -56.278 -56.33 53.04 52.9
5 9.906 -51.798 -51.797 88.83 88.57
10 9.892 -45.823 -45.831 177.6 177.1
20 9.870 -39.814 -39.823 355.3 354.4
30 9.8574 -36.294 -36.307 532.1 531.1
50 9.8698 -31.86 -31.867 886.8 885.2
100 9.8735 -25.843 -25.847 1772 1769
150 9.8734 -22.316 -22.32 2656 2652
200 9.8665 -19.819 -19.826 3542 3539
300 9.8744 -16.295 -16.301 5313 5308
The SSB carrier sideband ratio is derived by SSB[dBc] = (USB[dBm]+LSB[dBm])/2 - Carrier[dBm]
This measurement suggests that 10^(dBc/20) and Vrms has a linear relationship. (Attachment 1)
The data points were fitted by the function y= a x.
=> 10^dBc(SSB)/20 = 108*Vrms (@10.346dBm input)
Now we want to confirm this calibration.
DS345 @30.2MHz was modulated with the DC offset + random noise. The resulting AM modulated RF was checked with the network analyzer and the RFAM detector
in order to compare the calibrated dBc/Hz curves.
A) SR785 was set to produce random noise
B) Brought 2nd DS345 just to produce the DC offset of -2.52V (Offset display -1.26V)
Those two are added (A-B) by an SR560 (DC coupling, G=+1, 50 Ohm out).
The output was fed to Ext AM in DS345(#1)
DS345(#1) was set to 30.2MHz 16dBm => The measured output power was 10.3dBm.
On the network analyzer the carrier power at 30.2MHz was 9.89dBm
Measurement 1) SR785 1.6kHz span 30mV random noise (observed flat AM noise)
Measurement 2) SR785 12.8kHz span 100mV random noise (observed flat AM noise)
Measurement 3) SR785 102.4kHz span 300mV random noise (observed cut off of the AM modulation due to the BW of DS345)
The comparison plot is attached as Attachment 2. Note that those three measurements are independent and are not supposed to match each other.
The network analyzer result and RFAM measurement unit output should agree if the calibration is correct. In fact they do agree well.
|