[Skimmertalk] RP-16 front end tenderness

[Ted Gisske]

Repost, as the list didn’t like Bob’s pictures…

Ted

 

From: Ted Gisske [mailto:gisske at offex.com] 
Sent: Thursday, July 11, 2019 3:18 PM
To: 'Bob Wilson, N6TV'; 'SkimmerTalk Reflector'
Subject: RE: [Skimmertalk] RP-16 front end tenderness

 

Bob,

 

https://www.redpitaya.com/p52/stemlab-12288-16-sdr-kit-basic

 

Scroll down to “RF Inputs”. 

“Absolute Max. Input volt range: DC max 50V (AC-coupled), 1 Vpp for RF”

 

My goal is to keep my skimmer up during contests. I’ve loaded all observed variations and bastardizations of K9IMM into my bad call list in Aggregator, so they don’t go to the RBN. I’ve been running my QS1R in contests for years with no problems. I do run my skimmer in a pole-shed on my farm, about 150’ from any transmitting antenna. I Wi-Fi link the skimmer back to the shack to go to the internet.

73,

Ted

K9IMM

 

From: Bob Wilson, N6TV [mailto:n6tv at arrl.net] 
Sent: Thursday, July 11, 2019 2:09 PM
To: SkimmerTalk Reflector; Ted Gisske K9IMM
Subject: Re: [Skimmertalk] RP-16 front end tenderness

 

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.

 

 

 

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.

 

 

  

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

 


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