R/E/P Community

Please login or register.

Login with username, password and session length
Advanced search  

Pages: [1]   Go Down

Author Topic: In Condenser Mic Design, is "RF Modulated" the same as "RF Biased"?  (Read 394 times)

karlengel

  • Newbie
  • *
  • Offline Offline
  • Posts: 3
  • Real Full Name: Karl Engel

Sorry if not high-end enough but I figured folks here would know.
The Sennheiser MKH range uses an RF Modulated design principle which sets it apart from FET designs as I understand it. Rode's NTG3 (and NTG8) are described as "external RF biased". Is this the same thing?
Logged

David Satz

  • Full Member
  • ***
  • Offline Offline
  • Posts: 122
Re: Is "RF Modulated" the same as "RF Biased" in mic design?
« Reply #1 on: February 22, 2019, 08:24:45 pm »

Yes. It means that the microphone contains a high-frequency oscillator circuit that includes the capacitive capsule as a tuning element. The frequency of this oscillator circuit deviates in proportion to momentary changes in sound pressure at the capsule. The output is then demodulated, as in an FM radio receiver, to obtain the audio-frequency signal.

I find R°de's terminology rather awkward. There's no such thing as an RF electret, so it isn't necessary to say both "external" and "RF-biased" at the same time; "RF-biased" implies "externally biased". "RF-modulated" vs. "FET" would be a somewhat false dichotomy, however, since an RF circuit can use FETs.

A better classification--the one that I see used most often in technical literature--is radio-frequency vs. audio-frequency circuitry. Of course the latter term is needed only when a contrast has to be drawn between the two approaches.

--best regards
Logged

klaus

  • Moderator
  • Hero Member
  • *****
  • Offline Offline
  • Posts: 1613
Re: Is "RF Modulated" the same as "RF Biased" in mic design?
« Reply #2 on: February 23, 2019, 05:32:25 am »

Thank you, David, for your precise, jargon-free explanations of technical processes on this forum.
It's refreshing to read your posts. They keep reminding me that in writing, less is always more.

Regarding the efficiency and fidelity of RF condensers: Does the modulation/de-modulation process and its associated oscillator circuit components add more sonic artifacts, compared to those that are inherent with the classic (DC-charged) condenser principle?

I have not had enough experience with RF mics to be able to tell.
Logged
Klaus Heyne
German Masterworks«
www.GermanMasterworks.com

David Satz

  • Full Member
  • ***
  • Offline Offline
  • Posts: 122
Re: In Condenser Mic Design, is "RF Modulated" the same as "RF Biased"?
« Reply #3 on: February 23, 2019, 12:56:23 pm »

Klaus, thank you for your kind words. I don't know much about this beyond the basics. There are possible causes of distortion in RF modulator/demodulator circuits that linear "baseband" circuits don't have. These can be minimized, though, and the capsule (so laden with artifacts of its own, if that is what you would call them) can still determine the sound quality of a microphone.

The "RF-ness" of a microphone describes only the circuitry immediately around its capsule. The rest of the amplifier can have any characteristics that audio design allows. Sennheiser uses the non-RF parts of the circuit in their MKH-series microphones to shape the frequency response of the microphones, for example. This lets them use capsule designs that would otherwise be considered "underdamped"; then they compensate electronically to flatten and broaden the frequency response.

Of course that can also be done in DC-polarized microphones, and is done in some cases as you know. The point is that the circuitry closer to the output side of an "RF" microphone would be familiar territory to anyone who works with more traditional condenser microphones.

--best regards
Logged

uwe ret

  • Jr. Member
  • **
  • Offline Offline
  • Posts: 51
  • Real Full Name: Uwe Sattler
Re: In Condenser Mic Design, is "RF Modulated" the same as "RF Biased"?
« Reply #4 on: February 23, 2019, 03:43:17 pm »

The major advantage of the RF-principle is the low impedance of the capsule. For example, a typical condenser microphone capsule with a capacitance of 50 pF has an impedance varying between 160 MΩ at 20 Hz and 160 kΩ at 20 kHz. The same capsule in a 8 MHz RF-bias environment has an impedance of less than 400 Ω!
This makes the transducer far less sensitive to interference, moisture and other contamination, and easier to interface with solid state circuit topologies. Since the frequency deviations caused by the sound induced capacitance changes are relatively small, any potential artifacts are negligible and far lower than possible with conventionally DC-biased alternatives. Even without exotic design the self noise of RF-biased microphones is less than what can be achieved with equivalent traditional DC-biased circuits.
Furthermore, RF-bias enables push-pull microphone designs, completely symmetrical and balanced from capsule to output, which nicely does cancel all nonlinear capsule distortions. 
A Sennheiser white paper written by their lead professional microphones development engineer and researcher Manfred Hibbing on the subject of RF-condenser microphones is available in PDF-format, but restrictions on this forum do not allow attachments in that format. Contact me for a copy at uwe5758@att.net
Logged
Pages: [1]   Go Up