ds1669 Can it be used on HF RF?

Hi, I use the ds1669 as an audio volume control in a receiver. The input to this chip is from the mixer and contains not only the downconverted audio but also some RF. My question is, can this chip attenuate RF (because of slow response of it's internals for example) but let audio pass through?

Hi,

we don´t know your frequency of interest.

But you can consult the datasheet on your own: "Cutoff frequency".

Klaus

KlausST said:

Hi,

we don´t know your frequency of interest.

But you can consult the datasheet on your own: "Cutoff frequency".

Klaus

Click to expand...

The RF is on the HF range 2-30MHz
I do not see anything in the datasheet, but mayby I am checking wrong

Rated cut-off frequency is 1MHz, reducing to about 100KHz as the resistance is increased.

I suspect this isn't a hard limit but due to the RC time constant of the output circuit and other internal capacitances. I wouldn't recommend it for RF even if it wasn't obsolete.

Brian.

Hi,

I do not see anything in the datasheet, but mayby I am checking wrong

Click to expand...

A search for "cutoff frequency" should immediately show the right place.
Then there is a remark with "No 13". (Not a good solution in my eyes. They should have the values included in the table)


Klaus

betwixt said:

Rated cut-off frequency is 1MHz, reducing to about 100KHz as the resistance is increased.

I suspect this isn't a hard limit but due to the RC time constant of the output circuit and other internal capacitances. I wouldn't recommend it for RF even if it wasn't obsolete.

Brian.

Click to expand...

Thanks! So it could act an an RF HF attenuator and still pass AF through

Hi,

I recommend to use a properly designed filter. RC, LC, damped LC...

You say you use the EPOT as audio volume control.
64 steps of linear attenuation isn´t very useful for audio. I expect at the low level end (silent music) huge steps..so that maybe the one setup is too loud, but next lower level is too silent.
..and opposite at the upper end: you need a lot of steps to recognize a difference in loudness.

Thus I recommend to use dedicated audio volume control ICs.
Or at least a digital pot with logarithmic characteristic.

Klaus

KlausST said:

Hi,

I recommend to use a properly designed filter. RC, LC, damped LC...

You say you use the EPOT as audio volume control.
64 steps of linear attenuation isn´t very useful for audio. I expect at the low level end (silent music) huge steps..so that maybe the one setup is too loud, but next lower level is too silent.
..and opposite at the upper end: you need a lot of steps to recognize a difference in loudness.

Thus I recommend to use dedicated audio volume control ICs.
Or at least a digital pot with logarithmic characteristic.

Klaus

Click to expand...

It is for a noisy HF receiver, it doesn't matter much. 64 steps are practically more than adequate since the total af lvel is not high at all (small 4cm speaker).
It it can control the AF but limit the RF to the following amplifier, that would be enough

What you propose is rather like saying a 741 op-amp is a low pass filter because it can't amplify high frequencies. It doesn't work that way, even though RF may be attenuated, it still causes problems internally by shifting DC levels. Try holding a mobile phone close to an audio pre-amp and see what happens, clearly the audio amp isn't capable of passing 1.8GHz (or higher) but you will certainly hear buzzing as the RF is pulsed on and off.

For what you want, a simple RC filter will work better and give a more predictable frequency roll-off.

The DS1669 also has the problem Klaus mentioned, the steps are linear so you won't get normal volume control action, you really need a logarithmic digital pot for audio use.

Brian.