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  1. #1
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    Default Vintage Radar Detectors

    Hi All,

    I have a good understanding of how a superhet receiver works, but I'd like to know a bit about how the very early detectors (that predate superhet radar detector design) worked. Can anyone provide or point me to an explaination? Maybe a block diagram?

    Thanks,

    PbFoot

  2. #2
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    Default Re: Vintage Radar Detectors


  3. #3
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    Default Re: Vintage Radar Detectors

    Basically, the old pre-superhet designs used a tuned microwave antenna/cavity containing a modulator diode and a detector diode. The detector diode basically converted the received signal to DC. A signal was applied to the modulator diode, basically shorting and unshorting the cavity.

    So, when a received signal was present, the detector diode would produce an output signal that was pulsed/modulated at the same rate that was applied to the modulator diode. This was either pumped through directly as the receive audio, or fed through a separate circuit to produce an alert.

    Just about all pre-superhetrodyne detectors used some variation of this method.


  4. #4
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    Default Re: Vintage Radar Detectors

    I usually feel quite intelligent until I read what Jimbonzzz writes.

  5. #5
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    Default Re: Vintage Radar Detectors

    Hey thanks! That's some great info!

    A couple more questions:

    Would such a design only allow a single band to be detected?

    It seems like the selectivity might be extremely poor. Am I right?

    What detectors used this design? Was it the original Radar Sentry, Escort with the analog signal strength meter?

    What is the name of this type of design?

    Thanks!

    - PbFoot

  6. #6
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    Default Re: Vintage Radar Detectors

    Quote Originally Posted by <<JAZZY>> View Post
    I usually feel quite intelligent until I read what Jimbonzzz writes.
    Im still feeling dizzy.. I think im gonna go lay down now.

  7. #7
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    Default Re: Vintage Radar Detectors

    I can't speak for the selectivity, that would depend on how narrow-band the filtering circuits are. But for a 2 band detector, there would be 2 separate tuner circuits. One tuned to X and one to K.

    The original Escort was superhet IIRC. It was only the very early model detectors that weren't superhet.
    If I'm passing you on the right, YOU are in the wrong lane!

    If speed kills, how come I'm still alive?

    Active Countermeasures: V1 3.858, Escort Redline, Beltronics STi-R+, LI Dual 7.1x CPU/8.7 Heads (front)
    Other/Backup Countermeasures: V1 3.813 (loaned to friend), Beltronics Pro RX65 M4 6.3
    Vehicle: 2002 Audi A4 1.8T Quattro
    LEO Toys: Kustom Pro Laser II & III
    Encounters/Saves August 2011: Radar 3/1, Laser 0/0


  8. #8
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    Default Re: Vintage Radar Detectors

    Quote Originally Posted by PbFoot View Post
    Would such a design only allow a single band to be detected?
    What Kpatz said: two separate diodes/cavities would be necessary for receiving two bands.

    Quote Originally Posted by PbFoot View Post
    It seems like the selectivity might be extremely poor. Am I right?
    Yes: the only type of filtering they had was the tuning of the cavity, so they were prone to out-of-band false alerts.

    Quote Originally Posted by PbFoot View Post
    What detectors used this design? Was it the original Radar Sentry, Escort with the analog signal strength meter?
    Almost all detectors prior to the original Escort (with the analog strength meter) used some variation of this design.

    The original Escort was the first X/K superhetrodyne radar detector. There was another superhetrodyne unit before the Escort, but it was only superhetrodyne for X-Band.

    Quote Originally Posted by PbFoot View Post
    What is the name of this type of design?
    This type of receiver was called a "crystal detector".

  9. #9
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    Default Re: Vintage Radar Detectors

    Quote Originally Posted by jimbonzzz View Post
    This type of receiver was called a "crystal detector".
    Is that like the "crystal set" AM radios we built as kids?

    Hmm... a crystal set RD... complete with the high-impedance earphone. Powered by the radar signal itself. No need for a cord.
    If I'm passing you on the right, YOU are in the wrong lane!

    If speed kills, how come I'm still alive?

    Active Countermeasures: V1 3.858, Escort Redline, Beltronics STi-R+, LI Dual 7.1x CPU/8.7 Heads (front)
    Other/Backup Countermeasures: V1 3.813 (loaned to friend), Beltronics Pro RX65 M4 6.3
    Vehicle: 2002 Audi A4 1.8T Quattro
    LEO Toys: Kustom Pro Laser II & III
    Encounters/Saves August 2011: Radar 3/1, Laser 0/0


  10. #10
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    Default Re: Vintage Radar Detectors

    Quote Originally Posted by jimbonzzz View Post
    Basically, the old pre-superhet designs used a tuned microwave antenna/cavity containing a modulator diode and a detector diode. The detector diode basically converted the received signal to DC. A signal was applied to the modulator diode, basically shorting and unshorting the cavity.

    So, when a received signal was present, the detector diode would produce an output signal that was pulsed/modulated at the same rate that was applied to the modulator diode. This was either pumped through directly as the receive audio, or fed through a separate circuit to produce an alert.

    Just about all pre-superhetrodyne detectors used some variation of this method.

    A couple more questions I thought of:

    Could you please explain in greater detail the function of the modulator diode? Specifically the part about shorting and unshorting the cavity. I think I understand, but I just want to make sure I have the idea clear.

    Also, on modern detectors, the LO is swept using a VCO. Are there any other methods that are used to sweep the LO. What is the difference between a VCO and VFO, or are they two words for the same thing?

    Thanks,

    PbFoot

 

 

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