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Fw: Fw: Fw: Fw: Re: [softrock40] mixers

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  • n7ve
    I presume you are referring to a softrock V5 here. Single ended op-amps can use the open circuit, high impedance mode (+ input), or the low impedance, short
    Message 1 of 2 , Mar 28, 2006
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      I presume you are referring to a softrock V5 here. Single ended
      op-amps can use the open circuit, high impedance mode (+ input), or
      the low impedance, short circuit mode (- input). The V5 uses the
      short circuit mode.

      Open circuit voltage and short circuit current are electrical "duals"
      of each other. The short circuit configuration will de-emphasis the
      frequency roll characteristics of the detector cap, thus creating a
      flatter frequency response. This is desirable for a wide band SDR
      receiver.

      Think of the + and - terminals of an op-amp operating in a feedback
      configuration as having a short circuit between the two. Notice that
      the + side is biased to Vcc/2. Thus, the output voltage will try to
      "steer" the - input via the feedback resistor so that the input
      voltage from the 10 ohm input and the input from the feedback resistor
      cancel each other and leaves the - input at Vcc/2 also. This
      "steering" function through the feedback resistor is what creates the
      inverting signal gain.

      Thus, unless the op-amp saturates at one of the two output rails, the
      - input will always be forced to Vcc/2. Thus, any residual change on
      the detector caps, (and they will be small since we are dealing with
      detector currents here, not voltages) will disappear very rapidly.

      Operating at Vcc/2 is very desirable since that gives us the highest
      possible pk-pk output before distorting.

      - Dan, N7VE


      --- In softrock40@yahoogroups.com, windy10605@... wrote:
      >
      > Doesn't this low 10 ohm imput impedance also mean the capacitor charging
      > becomes part of a voltage divider network making the DC bias level for
      > the SR series around Vcc/4 vs the optimum (for higher dynamic range)
      > Vcc/2 ? ....divider applies to the audio detected too ? ....the RF was
      > mostly bypassed to ground with the capacitor as part of the
      > LPF/Integrating Detector.
      >
      > 73 Kees K5BCQ
      >
      > ----- Forwarded Message -----
      > From: windy10605@...
      > To: softrock40@yahoogroups.com
      > Date: Tue, 28 Mar 2006 16:41:07 -0600
      > Subject: Fw: Fw: Fw: Re: [softrock40] mixers
      >
      > What if the input impedance resistor on the Op Amp is 10 ohms like
      on the
      > SoftRock series ?
      >
      > 73 Kees K5BCQ
      >
      >
      > dan.Tayloe@...>
      > To: softrock40@yahoogroups.com
      > Date: Tue, 28 Mar 2006 22:19:07 -0000
      > Subject: Fw: Fw: Re: [softrock40] mixers
      > Message-ID: <e0ccor+7v35@egroups.com>
      >
      > 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 softrock40@yahoogroups.com, windy10605@ wrote:
      > >
      > > My comments were in reference to what is happening to the DC bias
      > voltage
      > > 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: softrock40@yahoogroups.com
      > > Date: Tue, 28 Mar 2006 19:52:36 -0000
      > > Subject: Fw: Re: [softrock40] mixers
      > > Message-ID: <e0c464+8hnu@egroups.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
      > filter.
      > >
      > > 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 softrock40@yahoogroups.com, 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: softrock40@yahoogroups.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
      > > problem?
      > > >
      > > > 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
      > > 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--
      > > > gain).
      > > >
      > > > 73 Kees K5BCQ
      > > >
      > > >
      > > >
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