R
Ross Herbert
- Jan 1, 1970
- 0
On Sat, 03 Jan 2009 22:51:18 -0900, [email protected] (Floyd L. Davidson) wrote:
:>"Ross Herbert"
:>
:>>
:>> Since the application is merely detecting signal "voltage" it hardly
:>> matters
:>> that the secondary impedance of the transformer is 600 ohms and the input
:>> impedance of the sound card is more like 10Kohms.
:>
:>** Fact is, it will be an advantage as the load across the phone line
:>needs to be high as possible so the level there is not affected.
:
:No, it merely needs to be "high enough", not "high as
ossible". If the bridging device has ten times the
:circuit impedance, there will be a 0.3 dB drop in levels
n the circuit, and a 30 dB loss across the bridging
:device.
:
:That is considered satisfactory for virually all
:applications. In this instance it would probably be
:satisfactory if the bridging impedance where half that.
:
:Regardless, you 10k Ohm value is absurd.
:
:>> The only reason one tries to
:>> match impedances is where one needs to maximise "power transfer" and that
:>> doesn't apply in this case.
:>
:>** Nonsense.
:>
:>Audio and other wide band transformers have rated ( ie optimum) source and
:>load impedances.
:>
:>This relates *directly* to them both operating efficiently AND exhibiting
:>their specified frequency response curve.
:>
:>Eg, a 600:600 ohm type operating into 10 kohms may well have a large
:>response peak at high frequencies while a 10k:10k type operating into 600
:>ohms will likely have serious roll off from a few kHz upwards.
:
:So read what you just wrote, and tell us how that affects a telecom
:circuit that is specified at 400-2800 kHz?
Should that not be Hz?
If so, voice frequency circuits were all 300 - 3400Hz in my day.
Phil is just being a bit too specific. My remarks were applicable specifically
to the VF telephony freq range, which is the area of interest to the OP.
:
:>With transformers that have 1:10 or 1:20 step-up ratios for mic-input to a
:>valve stage, both load and source impedances become quite critical just to
:>stay within a +/- 2dB corridor across the audio band.
:
:At perhaps 15-20 kHz. So just who cares, in this case?
Exactly!
:>"Ross Herbert"
:>
:>>
:>> Since the application is merely detecting signal "voltage" it hardly
:>> matters
:>> that the secondary impedance of the transformer is 600 ohms and the input
:>> impedance of the sound card is more like 10Kohms.
:>
:>** Fact is, it will be an advantage as the load across the phone line
:>needs to be high as possible so the level there is not affected.
:
:No, it merely needs to be "high enough", not "high as
ossible". If the bridging device has ten times the
:circuit impedance, there will be a 0.3 dB drop in levels
n the circuit, and a 30 dB loss across the bridging
:device.
:
:That is considered satisfactory for virually all
:applications. In this instance it would probably be
:satisfactory if the bridging impedance where half that.
:
:Regardless, you 10k Ohm value is absurd.
:
:>> The only reason one tries to
:>> match impedances is where one needs to maximise "power transfer" and that
:>> doesn't apply in this case.
:>
:>** Nonsense.
:>
:>Audio and other wide band transformers have rated ( ie optimum) source and
:>load impedances.
:>
:>This relates *directly* to them both operating efficiently AND exhibiting
:>their specified frequency response curve.
:>
:>Eg, a 600:600 ohm type operating into 10 kohms may well have a large
:>response peak at high frequencies while a 10k:10k type operating into 600
:>ohms will likely have serious roll off from a few kHz upwards.
:
:So read what you just wrote, and tell us how that affects a telecom
:circuit that is specified at 400-2800 kHz?
Should that not be Hz?
If so, voice frequency circuits were all 300 - 3400Hz in my day.
Phil is just being a bit too specific. My remarks were applicable specifically
to the VF telephony freq range, which is the area of interest to the OP.
:
:>With transformers that have 1:10 or 1:20 step-up ratios for mic-input to a
:>valve stage, both load and source impedances become quite critical just to
:>stay within a +/- 2dB corridor across the audio band.
:
:At perhaps 15-20 kHz. So just who cares, in this case?
Exactly!