[time-nuts] DIY VNA design, directional coupler

Alex Pummer alex at pcscons.com
Sun Aug 21 13:04:10 EDT 2016

directional coupler/circulator could be made with high bandwidth [ up to 
1GHz ] operational amplifiers, that circulator will work from DC..

driving A/D converter input asymmetrically; drive trough a 
Guanella-choke, but match the output of the choke



On 8/21/2016 3:21 AM, Attila Kinali wrote:
> On Sun, 21 Aug 2016 04:46:10 +0000 (UTC)
> Bob Albert via time-nuts <time-nuts at febo.com> wrote:
>> I was interested in this, but my needs are mostly below 100 MHz.
>> I wonder what could be done similarly for this lower range...
> As Orin mentioned, there are some designs for that range out there,
> best known are probably the two Orin listed (N2PK and the VNAW by DG8SAQ).
> Although these are good designs, they are not as good as the one by
> Henrik Forstén. Henrik addresses many issues that the other leave out
> for simplicity.
> What I would do instead is use Henrik's design and do some adaptions.
> There are four parts that limit the frequency at the lower end:
> the signal sources, the filters for the sources, the mixer and
> the directional couplers.
> For the signal source there are two choices: DDS and down-mixing.
> The DDS is probably the obvious choice and delivers good results,
> but limits the maximum frequency if you have price limit.
> The down-mixing approach uses one of the PLL's with VCO as the
> original design uses, but only within a limited range, eg around
> 200MHz. This signal can then be down-mixed using a crystal oscillator
> (or another PLL+VCO) and a suitable mixer (eg LTC5512 or a DIY diode mixer).
> Advantage of this is, that the spurs of the PLL+VCO can be surpressed
> to a large extend, as the frequency range is quite narrow relative to
> the output frequency of the PLL+VCO.
> For the directional couplers, the approach used with Henriks design
> will not work for low frequencies, as this type of coupler needs a length
> of approximately lambda/4 to work optimally. I.e. they would become
> unweildingly large. The two choices I am aware of for the lower frequency
> ranges are transformer based directional couplers or resistive bridges.
> Transformer based couplers have the disadvantage of a non-flat frequency
> response and an upper and lower frequency limit, given by the characteristics
> of the transformer (number of windings/inductance and the used ferrite).
> Their advantage is that they have very little loss. Resistive bridges on
> the other hand have a loss of 3db (respectively a -6dB signal at each output),
> but are totally flat down to DC and up to several hundred MHz or even GHz if
> RF resistors are used.
> Most of the above mentioned methods have a lower frequency limit somewhere in
> the range of 20kHz and ~100kHz. If you want to go below that limit, you will
> need to adapt the circuit further:
> For the signal source the DDS approach is the only one that will result
> in a good SNR at a reasonable price. Easiest way to go is to use a 16bit
> DAC at >1MHz and an uC or FPGA to feed it (but use some low jitter oscillator
> as clock source for the DAC). The other components in the signal path
> that are limiting are the baluns and mixers. I would get rid of those two
> all-together and digitize the signal from the directional couplers directly
> using an ADC with >1Msps and 16-18bit. If you limit yourself to the range
> of 10Hz-20kHz, you can do all this using audio ADC/DACs and get a very
> high performing system.
> 				Attila Kinali

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