[Elecraft] Re: K3 receiver sensitivity on 50MHz

[w2bvh]

An interesting design to benchmark against might be YU1AW's moderate 
noise figure / high IP3 preamps using BFP196 / BFG540. See 
http://www.qsl.net/yu1aw/bfp196peng.htm for some details.

73,
Lenny W2BVH
K2 sn 1520

Ian White GM3SEK wrote:
> Wayne
> wrote:
>> Ian White GM3SEK wrote:
>>
>>> On the subject of 50MHz, Wayne, please could you say more about the 
>>> receiver sensitivity specification?
>>> At present the sensitivity spec is the same on all bands, namely: 
>>> "-136 dBm (typical), preamp on, 500Hz b/w"....
>>
>> Hi Ian,
>>
>> The K3 is optimized for high dynamic range, which requires a 
>> high-intercept preamp and mixer as well as the narrowest possible 
>> band-pass filters (consistent with good return loss). We also used 
>> PIN diode T-R switching to eliminate relays during QSK. There are 
>> small front-end losses associated with each of these design choices.
>>
>> However, there is a way that one could have the best of both worlds. 
>> Those who would like an extremely low NF on 6 meters (or other bands) 
>> could patch in an external low-noise preamp between the RX ANT IN and 
>> OUT jacks. I've asked one our staff engineers to look into such a 
>> design, although there may be third-party products already available.
>>
>> You'd need the KXV3 option. The KXV3's RX ANT IN/OUT jacks are fully 
>> isolated from the transmit path, so high-sensitivity devices could be 
>> used in the preamp. The RX ANT IN/OUT jacks are also pre-BPF and can 
>> be turned on/off per-band. A 6-meter-only implementation with its own 
>> optimized BPFs could provide a world-class NF.
>>
>> Further, you could power the external preamp from the K3's 12-V DC 
>> accessory output (up to 0.5 A, switched).
>>
>> I personally prefer this means of achieving a very low 6-meter NF, 
>> because it doesn't result in compromises to the rest of the design. 
>> There are other rigs on the market that incorporate a second, 
>> switchable preamp specifically for this purpose, but none of these 
>> can match the K3's close-in dynamic range on HF.
>>
>> 73,
>> Wayne
>> N6KR
>
> Thank you for the prompt and thoughtful reply.
>
> Nobody should join a mindless stampede for the lowest possible NF on 6 
> metres. For more than 20 years, I have been urging VHF/UHF DXers to 
> optimize their dynamic range in exactly the same way that we do on HF.
>
> The receiver noise floor at VHF needs to be lower than at HF, to take 
> advantage of the lower antenna noise level; but it's a very delicate 
> tradeoff. Above all, it is vital to use no more preamp gain than 
> absolutely necessary, because unnecessary gain will have a dB-for-dB 
> impact on the strong-signal handling.
>
> The optimum solution is always to use a preamp at the antenna; but 
> that isn't always practical, which is why most transceivers offer the 
> option of a built-in preamp. The band-definable RX IN/OUT jacks on the 
> KXV3 will allow us to use a dedicated 6m preamp - if not actually 
> inside the rig, at least right behind it.
>
> Should Elecraft supply a 6m preamp as an option for the K3? Yes, I 
> think you should, because third-party preamps are notorious for having 
> far too much gain. To preserve the K3's reputation for high dynamic 
> range on *all* bands, it would be best to keep the gain/NF tradeoffs 
> for 6m under factory control. Obviously this option is not a high 
> priority; but it certainly would be worthwhile.
>
>
>