This forum is run by the BATC (British Amateur Television Club), it is service made freely available to all interested parties, please do not abuse this privilege.
I carried out some measurements of the output from my Portsdown 2019 on its own and with a newly built amplifier. Signals were 71MHz 333ks DVB-S. My low pass filter was used as the harmonics were otherwise across the span.
I'm new to spectrum analysis so please may I have some help interpreting the displayed trace?
1) Portsdown lime see only. No attenuation. 2) Portsdown 2019 plus minikits amp. 4.9v bias for 10watts on a regular moving coil power meter. 40dB attenuator in line. (It was c. 4W at the recommended 4.5v bias) Power, Bandwidth and shoulders? Role of the reference level?
The shoulders out of the Lime seem to be about -38 dB (you may find that you could improve this by changing the Lime Gain). The bandwidth is exactly what I would expect. Note that the major contributor to your shoulders is the subsequent PA, so probably not worth playing with the Lime Gain.
After the amplifier, the shoulders are at about -25 dB. Probably driving it at the limit of what you want, as although the first shoulders are in-band, the second shoulders (at -40 dB) are out of band. With a resolution bandwidth of 30 kHz, the carrier level would be (signal width/resolution bandwidth) 10 times (10 dB) higher, so those shoulders are actually -50dB on your transmitted power - which is a sensible limit.
Power meters (even regular diode/moving coil) are generally more accurate than spectrum analysers for power measurement, so I would go with that.
Looks good to go on the air!
Thank you for the reply. There's a good topic for a club video in this - in the style of Alan W2AEW.
Below are some pictures of follow on adjustments. I reduced the bias voltage from 4.87 to 4.59 V. I think that improved the signal in terms of bandwidth and shoulder separation. What do you see?
I have noticed the above mentioned dropout myself on 23cms , as have other local hams I know. Its an ongoing investigation I think and one of our excuses for constant and persistent testing. You might find this useful if you havn't already seen it. I'm sure it may have changed since more recent software updates.
We are slowly working our way through it to see what works and what doesn't for us.
One interesting thing is that some things DO GO using a C310 webcam, that DON'T GO using a C920. It's all interesting stuff.
Thanks to the Portsdown development team for all the fun they have provided. So much is owed by the many to the few.
Good luck with the testing... Ken
Thanks for your patience. It won't surprise you to know that the focus of my development work at the moment is overcoming this dropout problem.
Not sure that Portsdown will ever be compatible with all webcams. I'm concentrating on the C920.
Is my last spectrum better? I added a QRP-Labs 4m low pass filter on the amp input because I had one, it did no harm and thought there was an improvement in the displayed signal. This £4 filter's characteristic is shown in the Wiki filters section.
I think the wide shoulders are -48dBm down from the peak signal, the main signal is about 500kHz wide and the 1st shoulders are c. -30dBm down and 1MHz wide? I hope for narrower with DVB-S2, lower symbol rates and maybe H265.
I suspect that power measurement is the preserve of a power meter not one of these analysers and the 40dB attenuation simply scales the signal amplitude perfectly to fit it within the 200mW limit of my unit without altering its bandwidth???? True or False?
I shall sit down to read Agilent application note 150 https://www.keysight.com/upload/cmc_upl ... 0292EN.pdf for my own education and rewatch W2AEW's reliable Youtube films.
The last spectrum is better, but probably not because of the filter's frequency filtering characteristics. More likely that it has some loss, and so you are not driving the PA so hard.
DVB-S2 will be the same width (for the same symbol rate) as DVB-S. The use of MPEG-2, H264 or H265 will not affect the signal width, but will affect the quality of picture that you can fit in that bandwidth.
Not sure about the attenuation on the analyser. I am always very cautious of putting anything over 10mW near the input of an analyser, and use external attenuators to achieve that. However, more modern analysers might be more tolerant than the 1970 model sat on my bench!
Time to put the TX on an aerial I think!
I think I'll make the 4m cubic quad antenna in the current edition of Radcom February 2020. I've a delta loop at the moment from my past SOTA activity. See GW7AAV website for details.
Arrange some schedules with the other 4m DATV ops and head for the high car parks!