[hpsdr] Questions re internal and external LPF on Penelope, Penny Lane and Hermes

Wed Nov 7 03:27:28 PST 2012

Hi John.

As I mentioned, I have never measured the output from Penny so today I did  
it and found that the output is much cleaner than I measured from my  
Hermes. I also found that the output from Penny is less than from Hermes  
so I tested Hermes again. Then I found that if I reduced the output to the  
same level as from Penny, the spectrum was also the same. I was driving  
Hermes too hard, around 600 mW PEP but at 500 mW as the specifications  
are, the output is clean and all harmonics are > -50 dBc up to 17 m and >  
40 dBc above.

I have experimented with the gain and also some filtering in this Hermes  
so I have obviously left it with a higher than necessary gain in the last  
stage. The Gain Per Band settings I ended up with today without over  
driving the PA is between 41 and 43 dB.

I do not have any of the production boards to measure on so if someone can  
do that on one, we can have this results confirmed.

If you use the two-tone test in PowerSDR and measure with a good Spectrum  
Analyzer (or Mercury or another Hermes) adjust the GPB settings until the  
IM3 products are just visible at >-50 dBc. Then do a broadband measurement  
and look for the 2.nd and 3.rd harmonic on the different bands.

I am sorry for this miss yesterday but I am trying to learn a new CAD  
program so this "multitasking" must have made me dull!

73, Kjell

På Wed, 07 Nov 2012 11:13:18 +0100, skrev John Marvin  
<jm-hpsdr at themarvins.org>:

>   Hi Kjell,
>
> Thanks for you response. I realize that any time a PA is used that all  
> bets are off, and almost certainly an appropriate external LPF will be  
> needed.
>
> You mentioned that the 2nd and 3rd harmonics are only -30 dBc on 20  
> meters.  This means a LPF will be required even without a PA, unless I  
> can claim I installed my Hermes prior to Jan 1, 2003 :).  Oh well, I  
> guess I'll have to wait until I've had the time to hack my amp before I  
> can test the transmit path.
>
> I'm assuming that the harmonics on 20 meter were better than -43dB on  
> Penelope. I'm a little surprised things are so much worse on Hermes, but  
> ultimately that is not really important since the solution is simple and  
> fairly standard.
>
> Thanks,
>
> John
> AC0ZG
>
> On 11/6/2012 3:30 AM, Kjell Karlsen wrote:
>> Hi John.
>>
>> I have been following the development of Hermes and have done the  
>> measurements of the output spectrum. I found that the output from the  
>> PA had too much noise and spurious due to pick up of direct and  
>> harmonics of the clock oscillators by the amplifier. B
>>
>> The bandwidth of the PA is very high. To limit the bandwidth, 2  
>> capacitors were inserted on U7 inputs.
>>
>> In addition, LFCV-52+ was included after the PA. After this  
>> modifications, the alias from 50- 52 MHz, is >-50 dBc, at 53 MHz it is  
>> -45 dBc and at 54 MHz it is -40 dBc. The spurs and noise are very low,  
>> >60 dB down except the 2.nd harmonic that is -40 dBc at 51 MHz. This  
>> means that Hermes can be connected to the antenna  without any extra  
>> outboard filtering at 6 meter.
>>
>> On the lower bands, the harmonics are on not suppressed by the LPF on  
>> the output so be careful. On 20 meter, the 2.nd and 3.rd harmonics are  
>> only -30 dBc.
>>
>> If a PA is connected, a LP filter is needed. Harmonics are always  
>> generated by the PA and must be removed to meet the FCC requirements.
>>
>> It is not possible to use Hermes on higher order alias frequencies  
>> without taking the signal directly from the DAC. There are no jumpers  
>> or other provisions on the PCB to do so. FL1 must be removed to get  
>> access to T1.
>>
>> The same is with the RX. There is no way of direct connection to the  
>> Pre-amp without removing FL6.
>>
>> I hope this answers most of your questions concerning Hermes. I have  
>> not measured on Penny or Penny Lane.
>>
>> By the way, we are working on a new and hopefully improved PA for a  
>> future "Hermes PRO"
>>
>> 73, Kjell
>>
>>
>>
>> På Tue, 06 Nov 2012 05:53:00 +0100, skrev John Marvin  
>> <jm-hpsdr at themarvins.org>:
>>
>>> ***** High Performance Software Defined Radio Discussion List *****
>>>
>>>   I was looking at a FAQ on Penelope that addressed why Penelope has  
>>> no filtering on the output. That got me interested in looking at how  
>>> things have changed going from Penelope to Penny Lane to Hermes:
>>>
>>> 1) Without any filtering I would think that there would have been a  
>>> fair amount of aliased output. In some cases people use that as a  
>>> feature, i.e. deliberately use the aliasing to generate desired  
>>> signals in ranges above the Nyquist rate. That was even mentioned as a  
>>> possible feature of Penelope.  But at the same time it was claimed  
>>> that at 0.5 W output, Penny Lane meets the FCC requirements without  
>>> additional filtering. How far down was the signal in the Fs/2 - Fs  
>>> range compared to the DC - Fs/2 range?
>>>
>>> 2) So, after all the good arguments for no filtering, two filters were  
>>> added to Penny Lane, an RLP-40+ and what appears to be series LC  
>>> band-stop discrete filter. Were the original arguments not valid, or  
>>> was there another reason for doing this?
>>>
>>> 3) Then yet another LPF filter was added to Hermes (an LFCV-52+). Why  
>>> was this added?
>>>
>>> 4) The Penelope FAQ claimed that no external filtering was needed at  
>>> 0.5watt, yet the user guide for Hermes claims that an external LPF is  
>>> always needed, even with all the additional filtering.  Is that  
>>> actually true? If so, what are the sources of out of band signals that  
>>> exceed the FCC requirements that didn't exist on Penelope?
>>>
>>> 5) Has anyone done any detailed measurements on Hermes TX output?
>>>
>>> 6) Has anyone ever tried doing VHF/UHF receive and/or transmit work on  
>>> Mercury and or Penelope/Penny Lane using undersampling/aliasing  
>>> techniques? If so, how and what was done (e.g. filters bypassed, FPGA  
>>> work done, etc.).  Has there ever been any consideration of adding  
>>> jumpers or other features to make it easier to bypass the input and  
>>> output LPF filters in order to explore this possibility (which would  
>>> require external band pass filtering)?
>>>
>>> Thanks!
>>>
>>> John
>>> AC0ZG
>>>
>>>
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>>>
>>
>>
>
>

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