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

[John Marvin]

  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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