[Skimmertalk] RP-16 front end tenderness

[Bob Wilson, N6TV]

I wish this reflector would allow posting of small images or attachments (I
think it's a setting that can be changed).

I'm resending my last post without images (see below).  A copy with images
was sent directly to K9IMM.

73,
Bob, N6TV

On Thu, Jul 11, 2019 at 12:08 PM Bob Wilson, N6TV <n6tv at arrl.net> wrote:

> On Thu, Jul 11, 2019 at 7:26 AM Ted Gisske <gisske at offex.com> wrote:
>
>> Front-end protection for your new Red Pitaya is a worthwhile investment.
>> RP
>> specs the maximum input voltage at 1V P-P.
>
>
> Ted,
>
> In which RP spec did you find this number?
>
> I'm looking at https://www.redpitaya.com/n86/new-stemlab-sdr, and under *Technical
> Specifications* it says the "full scale" RF input range is 0.5 Vpp / -2
> dBm, but it gives no maximum input voltage.  I assume it just clips over
> 0.5 V.
>
> <image removed>
>
> The only mention of "1V P-P" is under the RF *Output* section (because
> the SDR 122.88-16 is also a signal generator).  It says Full-scale RF
> *output*:  1 Vpp / +4 dbM.
>
> <image removed>
>
>
>> This is probably so low due to the 1:14 input transformers.
>
>
> They are not using a 14:1 transformer in this board.
>
> I found a "Development" schematic for the STEMlab SDR 122-16 here:
>
> https://redpitaya.readthedocs.io/en/latest/developerGuide/122-16/shem.html
>
>
> The input transformer (TRF1) is listed as an MABA-007159.  The datatsheet
> <https://cdn.macom.com/datasheets/MABA-007159-000000.pdf> specifies that
> it is a "1:1 Transmission Line Transformer" with maximum DC (?) power of
> 250 mW -or- maximum DC (?) current of 30 ma.  In a 50 ohm system, the
> former would be 3.5 VDC and the latter 1.5 VDC.  1.5 VDC is equivalent to
> 2.36 V RMS AC, I think, much higher than 1 V P-P.
>
> The next transformer (TRF3) is a WBC4-1TLB.  The datasheet
> <https://www.mouser.com/datasheet/2/597/wbc-463492.pdf> specifies that it
> is 4:1 "Wideband Transformer", 1/4 Watt (250 mW) input power, 250 ma max
> current rating, 300Vrms (!) interwinding isolation.
>
> After that there is an impedance bridge made out of resistors that feeds
> the input of the 16-bit ADC converter.  Unfortunately, they don't list the
> ADC part number, and it seems to be covered up by the heat sink.  I assume
> it can handle much more than 1 V P-P.
>
> The DXE protector does a great job of limiting this voltage to 2V P-P.
>> That, however, is twice the rated max voltage of the RP.  Not good enuf...
>>
>
> It may indeed be good enough.  2V P-P into 50 ohms is only 80 mW.
>
> In sum, I don't think adding extra front end protection is really
> necessary, unless you can measure 250 mW or more of RF input to the
> receiver when you are transmitting, after the Front End Protector and
> preamp.
>
> My transmit antennas are directly above my Skimmer antennas (small
> suburban lot here).  Before with the old RP-14 I used a DXE Front End
> Protector on the Input to the 11 dB Clifton Labs Preamp, but never added a
> second FEP between the pre-amp and the external 14:1 transformer.  I've
> transmitted many times (but not for long) with full power, and the RPs
> continue to perform fine after I stop transmitting.
>
> Of course, I've long since concluded that it is best to completely shut
> down all skimming (or at least posts to the RBN) while operating full power
> for long periods, such as in a contest.  There are just way too many
> spurious signals generated by a nearby transmitter, overwhelming the
> skimmer software and CPU (especially if RTTY Skimming).
>
> If your goal is to be able to operate a contest while keeping your
> skimmers going full time at the same location, I think you may be
> disappointed with all the "self-spots" that are generated, spots that might
> get you disqualified from the contest (even if they are all "busted" spots
> like K9SMM, MK9IMM, etc.).
>
> 73,
> Bob, N6TV
>