Soft Rock V5 - Spice Analysis of 20 meter quadrature generation


This page shows my results modeling a NJ QRP DDS card driving the SR 40 V5 quadrature genration circuit.

To do the modeling, I used LT Spice from Linear Technologies. This is a very nice free download of a GUI enabled Spice that runs on Windows. Modeling this little circuit gave me a reason to figure out how to use this program. A good addition to my circuit analysis toolkit.


The circuit I modeled is shown below. The Spice Model can be downloaded here.

The DDS card is modeled as a 1Vp-p 14 MHz sine wave voltage source with 50 ohm series resistance. Rmatch is a resistor I added to match the DDS card to the Soft Rock input.


The plot below shows the simulation result for the circuit above which is essentially the standard Soft Rock V5 setup.

The plot below shows the simulation result for C2=100pF. Interestingly the phase relationships look about the same as they do for 47pF, so the value of the phase shift might not be as critical as orignally thought.

Let's see if it will work at C2=220pf. This time both startup and steady state plots are shown.

Interesting to see the startup transient on Vin2 - the input to the 2nd comparator. Takes it a few cycles to get to steady state. The steady state duty cycle on Vout2 is a bit long, but the radio would probably still hear, albeit with some reduced sensitivity.

An Anaomoly

One interesting thing I see in the simulation traces is Vin1 seems to be centered on 1.5v. I'd have suspected it to be at about 2.5v. Either there's an error in the simulation/model, or the comparator input is not a very hi impedance as it is assumend to be. Wonder which it is?

Comments to: Bill Tracey (

Last Updated 19 November 2005

Copyright © Bill Tracey 2005