[time-nuts] DDS on AD9852
time at radio.sent.com
Sat Jan 17 12:49:23 EST 2015
I'm new to this list, so please forgive me if I am missing some older
information not in your post.
I also believe you are overdriving the amplifier, but the oscilloscope
display scaling is confusing us. Several things stand out at me from
* The AD9858 has a current-mode D/A output. So I assume you must have
some transformer or amplifier on a development board between the
AD9858 output pin and the signal you show in the "2.jpg" image.
* The Rigol oscilloscope image "2.jpg" shows a 6.6 Vp-p signal (6.6
divisions p-p at 1 V/div).
* The other signal "1.jpg" or the apparently identical signal "3.jpg"
show a 33 Vp-p signal (6.6 divisions p-p at 5 V/div). This is far too
high a voltage to be realistically present at the output of that
little Ten-Tec amplifier.
* My guess is that you somehow have the scope fooled into thinking it
has a X10 probe attached, but have a X1 connection. So I'm guessing
that "2.jpg" is actually showing 660 mVp-p, and "1.jpg" is actually
showing 3.3 Vp-p.
* I can't find any detailed specs for the Ten-Tec model 1001 amplifier
(such as the schematic or which amplifier MMIC or transistor it
uses), but it appears to be rated for about 18 dB gain. That's a
voltage gain of around 8.
* With an input of 660 mVp-p, the Ten-Tec amplifier output swing would
be (0.66)(8) = 5.3 Vp-p (if the amplifier could swing that far). But
the amplifier is probably strongly overloaded, since it's designed
for small signals (such as a receiver preamplifier).
* So I think your "1.jpg" waveform is showing the output of an
*overdriven amplifier*. But without knowing the oscilloscope probing
details (was the oscilloscope connected using a matched 50 ohm cable
with proper termination at the scope end), oscilloscope vertical
scaling, and schematic of the model 1001 amplifier (including it's
power supply voltage) it's not possible to be certain.
* If you inserted a 6 dB attenuator (voltage ratio of 2X) at the input
of the amplifier, I would guess that the amplifier output would
appear reasonably close to a sinewave, although the amplifier might
still be in compression and generating significant harmonics. The
amplifier must be terminated in 50 ohms for it to work properly.
Bill Byrom N5BB
On Sat, Jan 17, 2015, at 01:51 AM, Tom Miller wrote:
> ----- Original Message ----- From: "d0ct0r" <time at patoka.org> To:
> "Discussion of precise time and frequency measurement"
> <time-nuts at febo.com> Sent: Saturday, January 17, 2015 12:02 AM
> Subject: [time-nuts] DDS on AD9852
>> I would like to ask some advise about the signal distortion I get
>> from my DDS project. The T-Bolt GPSDO is in use as a REF signal
>> source (10Mhz) for AD9858 DDS. This chip has two COS analog outputs
>> which opposite to each other. I am using 20x reference multiplier in
>> my project. And I am using Low Pass filter and RF amplifier connected
>> to ONE of the COS output. The other output has no filter or op-amp on
>> it. Attached is four files with OSC screen shots, where the signal
>> which goes through the filter and amplifier somehow distorted. In
>> contrary, the signal from another output (without filter and op-amp)
>> has near perfect curve. I am using the same cable for connection to
>> Oscilloscope.. And I set 5 Mhz output on DDS as an example. In this
>> my project I am using Ten-Tec broadband pre-amp. The filter is
>> standard Chebyshev Low Pass filter made by MINI-CIRCUITS. Model
>> PLP-90 (81 Mhz).
>> Here is the link to Ten-Tec
>> Is it normal to have that kind of distortions for the signal, or
>> something wrong with design ?
> You are overdriving the amplifier. What is the part number of Q1? It
> looks like a MAR-xx something.
> The Tentek page does not have any good technical details on the amp
> that I could find. What info do you have?
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