Fw: Fw: Re: [softrock40] mixers
- The time constant on the DC bias is almost a "don't care". As soon as
the signal stops, the residual charge freezes and becomes a gradually
decreasing DC charge. If the signal was at a 20 KHz offset, it
disappears immediately when the signal stops as a signal is necessary
to create the detected 20 KHz AC signal.
You would need a really large signal to worry about the bleed off time
constant of any residual DC charge.
- Dan, N7VE
--- In email@example.com, windy10605@... wrote:
> My comments were in reference to what is happening to the DC bias
> charge/discharge relative to the capacitor, not the AC signal voltage on
> top of it .....and how the cycling of the bias voltage, based on RC time
> constants, could cause problems if it is not allowed to go to --and
> stay-- at Vcc/2.
> 73 Kees K5BCQ
> ----- Forwarded Message -----
> From: "n7ve" <dan.Tayloe@...>
> To: firstname.lastname@example.org
> Date: Tue, 28 Mar 2006 19:52:36 -0000
> Subject: Fw: Re: [softrock40] mixers
> Message-ID: <email@example.com>
> This is why this is an "integrating" detector. Each RF 1/4 cycle
> sample contributes a bit of charge to the detection cap. Over time,
> the charge builds up to the average of the 1/4 RF input.
> It does not matter if there are many shorter pulses (28 MHz) for fewer
> longer pulses (7 MHz); the amount of total energy integrated onto the
> detection caps per unit time will be roughly the same. Thus thinking
> of this as an "IQSD" (Integrating Quadrature Sampling Detector) will
> help you understand the detector a bit more clearly.
> The detector frequency response is a function of the RF input
> impedance (50 ohms as modified by various input transformers and
> additional series R in the circuit) and the detection caps. This
> forms a simple R/C low pass filter. The actual sampling rate (7 MHz
> or 28 MHz) will not change the time constant of this R/C low pass
> Also, understand the averaging ("integrating") character of this
> detector and it becomes clear why averaging over 1/4 cycle of an RF
> sinewave produces a higher peak (0.9x) than over a longer period such
> as a 1/2 RF cycle (0.707x) and makes it obvious why averaging over 1
> full RF cycle will produce zero output. It also explains why the even
> order harmonics are suppressed (zero average) and the odd order
> harmonics produce output.
> Also, if you are not concerned about frequency roll off of the
> detector caps, a normal op-amp connected differentially across the
> detection caps of a detector provides a high impedance for the "+"
> input, and a short circuit across the "-" input. I think of these two
> modes as duals of each other. One treats the input signal as an open
> circuit voltage source, the other as a short circuit current source.
> It is a very unbalanced situation, but using the differential signal
> from two sources does effectively reduce the noise figure of the
> op-amp by a factor of two, which is quite useful if you are going for
> best sensitivity (half the output noise for the same net voltage gain).
> Instrumentation amps (INAs) have great input impedance, but they have
> to be run at a higher gain to realize a low input noise. INAs like to
> be run at 500x to 1000x, and all the gain that we really need for a
> post detector pre-amp is about 50x. Extra gain just makes the pre-amp
> more susceptible to large signal overload, which nobody wants.
> - Dan, N7VE
> --- In firstname.lastname@example.org, FRANCIS CARCIA <carcia@> wrote:
> > When everything is sitting at 2.5 volts no signal in the voltage on
> the transformer secondary effects the direction for each sample. It
> will result in a source or sink and the z of that signal should be
> constant. The time constant is fixed but the sample time goes down
> when the frequency goes up. This means it will take more samples to
> charge the time constant to a rail. This lag prevents the rc to peak
> charge. right???
> > windy10605@ wrote: If the charge time constant is always
> much less than the discharge time constant, the capacitors will charge
> up quickly to the 2.5V bias level. The time constant after the switch
> is fixed but the ....."time constant" into the capacitor, when
> including switch on/off characteristics, is not. In your example you
> will have 10 times 1/10 the sample period which should charge up to
> Vcc/2 for proper bias. The audio signal samples being read are 1/x00th
> of the RF frequency. Using a very high impedance Op Amp would be the
> best alternative and using a differential input would insure the bias
> levels presented are equal. Hey, that's what SDR-1000 does. Dang,
> Gerald Youngblood's FlexRadio group is sharp !
> > 73 Kees K5BCQ
> > carcia@>
> > To: email@example.com
> > Date: Tue, 28 Mar 2006 08:58:53 -0800 (PST)
> > Subject: Re: [softrock40] mixers
> > Message-ID: <20060328165854.3411.qmail@>
> > 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
> > windy10605@ wrote:
> > 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
> > the rail since the reference DC bias level is fixed at Vcc/2. This
> > fine at the lower frequencies but at higher frequencies this is more
> > difficult to do since the switch quadrature output time % drops a
> > 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
> > 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--
> > gain).
> > 73 Kees K5BCQ
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