Re: My PHSNA building & testing experience

Nick Kennedy
 



On Tue, Aug 23, 2016 at 10:19 PM, rex.vk3pk@... [PHSNA] <PHSNA@...> wrote:
 

Hi all,
I have just completed building &bench testing my PHSNA.  Packaging is still to be done.
I am using a DDS60 & a Mini Pro. & a level detector board by SV1AFN.

I made a couple of small changes.
a) I did not use a 100 Ohm trimpot as I did not have one on hand.   In addition, the output of the AD8307 is very unlikely to exceed +2.5 V (+15dBm), therefor I changed R1 & R2 on the regulator to 510 Ohm's to give me aAVref of 2.534.   The code was changed accordingly. After calibration, this gives a slop of 0.1056 approx  Ie. more counts/dB   That works good.


​Sounds good. I allowed for up to 2.6 V output (+18 dBm) and my slope is 0.0998.​
 


b)I used a SARK110 to do the calibration using the preset levels of -73.-63,-53,-43,33,-23,-13 & -10dBm
   After calibration it is very accurate around the higher levels, but gives an error of about -1.5 dBm at the -73 dBm. level.      Why is this so????


​A guess might be that you are near the noise floor, especially with the detector circuit not in a box yet.​
 


c) After calibration , I adjusted the gain of op-amp 1 on the DDS50 board. to give me an output level of -10dBm.  All good.  I did note that the input into op-amp 1 (190 mV p-p) & the output of op-amp1 (560mV p-p) is quite clean looking (using a Tektronix 100 Mhz. CRO),  however, the output of op-amp2 the output is far from sinusoidal (1024 mV p-p).  Is this normal.  The gain of op-amo 2 is about 4, so I would have expected about 2000 mV p-p and a clean looking sinusoid.   


​That definitely doesn't sound right. I also use a DDS-60 board and my output is a good looking sinusoid.
 


d)  I expect to run my gear of my station supply, which is 13.2V, so the regulator on the PHSNA board gets pretty hot.   So, I added a DC-DC pre-regulator to drop the V+ voltage to about 8.5 volts, which of course will be the Vsupply of the op-amp.   I would not have thought that the op-amp would be saturating at that supply voltage.    In fact, increasing the supply voltage made little difference to the look of the sinusoid.


​The manual for the DDS-60 says supply voltage can be 8 to 16 VDC, so you are in the right range.

73-

Nick, WA5BDU​
 




This may help some constructors.  In the mean time I would welcome comment on the distortion of the
sinusoid in op-amp2  and the level error near the lower calibration point.

Lee VK3PK  


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