Re: [softrock40] mixers
- nother simple mind at work....The time constant is fixed after the switch, pick a value.the voltage on the switch is 5 volts biased at 2.5 volts. A 5 volt peak to peak voltage signal would just fit through the switch if you ignor losses. Say at the lowest operating frequency a 5 Vp-p signal charges each rc to 5 volts peak. Now you increase the frequency by ten times. a 5 volt peak signal will not charge the same time constant to the peak value with 1/10 the sample period. Could this be part of the problem?
Just thinking out loud again......................
Ref my earlier notes ....seems like you need to allow an RC time
constant between the capacitor and the Op amp input impedance and the
capacitor and the charge impedance to allow the capacitor to charge up to
--and stay-- at the DC bias level .....or you drive the Op Amp output to
the rail since the reference DC bias level is fixed at Vcc/2. This works
fine at the lower frequencies but at higher frequencies this is more
difficult to do since the switch quadrature output time % drops a little
because the switch rise/fall times are "fixed". I think higher input
impedance Op amp configuration is worth a try on the SR-4, SR6 when used
above 18Mhz ? As a test, maybe a SR-7 Op Amp configuration using a 1K
input resistor and 50K feedback resistor (so you still have --some--
73 Kees K5BCQ