On 07/10/2012 07:42 PM, Steve Kerrison wrote:
On 10/07/12 17:20, Antti Palosaari wrote:
I am looking how to implement LNA support for the DVB API.
What we need to be configurable at least is: OFF, ON, AUTO.
There is LNAs that support variable gain and likely those will be
sooner or later. Actually I think there is already LNAs integrated to
the RF-tuner that offers adjustable gain. Also looking to NXP catalog
and you will see there is digital TV LNAs with adjustable gain.
Coming from that requirements are:
adjustable gain 0-xxx dB
LNA OFF
LNA ON
LNA AUTO
Setting LNA is easy but how to query capabilities of supported LNA
values? eg. this device has LNA which supports Gain=5dB, Gain=8dB, LNA
auto?
Without having a sample of device capabilities this question may be
irrelevant, but what if the gain is somewhat continuiously configurable
vs. discretized? For example can some be configured just for 5,8 and 11,
whilst some might have some 8-bit value that controls gain between 5 and
11?
Yes.
For example external DVB-T/C LNA NXP BGU7033. It does have 3 modes, 10
dB, 5 dB and bypass -2 dB.
LNAs offering more adjustable gain is called VGA (variable gain
amplifier). It could have control interface like 5-bits or some other
bus like I2C. Those are usually integrated to the silicon RF-tuner and
in that case configuration is possible via tuner registers. Usually it
is not needed to configure at all on run-time.
LNA ON (bypass) could be replaced with Gain=0 and LNA ON with Gain>0,
Gain=-1 is for auto example.
How should the API handle differences between the specified gain and the
capabilities of the LNA? Round to nearest possible config if it's within
the operating range; return error if out of range?
I think rounding is best approach.
Maybe it is not necessary to read at all. Just allow user set LNA in
steps of one number, 0, 1, 2, 3, ..., 100 and let the driver detect
scale. In that case simple LNA having only one static gain value has
values: AUTO, 0, 1. LNA having 2 gain levels: AUTO, 0, 1, 2. VGA having
3 bit control: AUTO, 0, 1, 2, 3, 4, 5, 6, 7.
regards
Antti
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http://palosaari.fi/
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