Preselector for HL2

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Ezio Insinna (Emi83170)

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Sep 6, 2023, 1:07:25 PM9/6/23
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SV1AFN has a nice preselector in his sales program. A nice addition to the HL2.

I've built it into an aluminium case and have added a 20dB attenuator as well as
a LNA from Nooelec. 
The latter are easy to connect, two SMA connectors each (in and output) are provided.
I've used a Switchbox from Elad with the addition of a reed relay to disconnect the preselector when transmitting. You can also use the MFJ 1708B which has additional contacts avoiding the extra reed relay.. 
By the way the preselector can be used with any SDR by the use of a splitter.

I've not made any measurements at the moment but I estimate the reception to be 
less sensitive to broadcast interference. Selectivity seems to be increased.
Measurements will follow later on.
Here some pictures . If you needed more info please contact me directly.

73 to all 

Ezio F5MWA / K3EK
IMG_20230904_180418_112.JPG
IMG_20230906_155601_114.JPG
IMG_20230904_180311_110.JPG
IMG_20230904_180450_113.JPG
IMG_20230904_180341_111.JPG

radi...@mail.com

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Sep 8, 2023, 5:04:04 PM9/8/23
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Don't know about others but I've never been aware of any issue at all with strong BC stations causing problems with the HL2, even more so with the N2ADR board present. In fact I would say the opposite. It's exceptionally resistant to strong BC problems? Anyone else seeing problems with stock HL2? I'm based in UK with plenty very strong HF BC stations in 40m band region from EU and also multi-kW MF station about 7 miles away from me.

Max


On Wednesday, 6 September 2023 at 18:07:25 UTC+1 eziomi...@gmail.com wrote:
SV1AFN has a nice preselector in his sales program. A nice addition to the HL2.

I've not made any measurements at the moment but I estimate the reception to be 
less sensitive to broadcast interference. Selectivity seems to be increased.


ron.ni...@gmail.com

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Sep 8, 2023, 6:08:28 PM9/8/23
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> Anyone else seeing problems with stock HL2?
I don't see problems from nearby high-power MF broadcast stations.  But I do see my HL2 overloaded, even when the gain is turned down, if a nearby amateur station operates aiming his HF beam antenna directly at my QTH.  A bandpass filter might at least allow me to use the HL2 on a different HF band than the HF QRM.  But I haven't yet found a suitable QRP bandpass filter kit yet with which to test that hypothesis.
73, Ron, n6ywu

Ezio Insinna

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Sep 9, 2023, 3:03:41 AM9/9/23
to ron.ni...@gmail.com, Hermes-Lite
Hello Max and Ron,

Max : You're right about the broadcast stations, the HL2 is rather resistant. However, I've noticed that the selectivity is increased not only on the Ham bands but also on the shortwave broadcast bands.
Switching in an additional attenuator (which can be programmable) is very handy.
Ron : I can see the HL2 easily overloaded with the LNA switched on on 40 meters. On other bands (from 17 meters onward and especially on 10 meters) the LNA is useful.
Finally I do not regret the addition  and I use the preselector with a splitter to feed any of my other SDR receptors.
I just wanted to inform other HL2 users.

73

Ezio F5MWA / K3EK




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Steve Haynal

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Sep 9, 2023, 2:48:30 PM9/9/23
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Hi All,

The HL2's AD9866 has a built-in LNA, -12dB to +48dB. I don't think it is necessary to add external attenuation and/or LNA.

The HL2 is a direct down/up conversion radio. This means you have a radio capable of a wide (~35MHz) spectrum. Why cripple and add expense/complexity by adding a preselector unless your QTH absolutely requires one? Any preselector confines you to a narrow bandwidth and makes things like skimming 10 HF bands concurrently much more difficult:

With a wideband radio, I believe thinking needs to change and focus on how to make more and better use of the bandwidth and digital data.

The N2ADR filter board includes a HPF which blocks AM radio stations. This is the main source of ADC overload for most locations, and something that can't be easily avoided in software or FPGA.

If people are interested in preselectors for IMD reasons, then this is a perfect opportunity to look at correcting that in software or more specifically FPGA code. Think PureSignal but for receive. You have access to large swaths of bandwidth and FPGA capabilities to process that data at speed. Instead of adding traditional filtering, consider developing some new software or FPGA based techniques.

73,

Steve
kf7o

radi...@mail.com

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Sep 9, 2023, 4:01:45 PM9/9/23
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I was not aware of the option to turn the LNA off, but as Steve says, it has a very wide gain range. On HF bands I operate (exclusively with SDR Console software) with settings around 0dB and never been aware of any overload at all. With the transverters I have (which are somewhat old) I sometimes turn the LNA up to perhaps 25dB if it's necessary although probably back to 0dB once I get decent antennas and masthead preamps at VHF/UHF. I'm actually compensating for gain in the wrong place at the moment! Never had the need to turn LNA down lower than 0dB on any band with my 132ft EFHW. HL2 just takes it all in it's stride. This is all with the N2ADR filter board switched in.

73

Max

Steve Haynal

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Sep 11, 2023, 12:46:05 AM9/11/23
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Hi All,

The LNA on the HL2 is not off and on, but can be set from -12dB to +48dB in 1dB steps. I tested last night and found that the difference between disabling and enabling the N2ADR filter to block AM signals provides me with an extra ~18dB of headroom. Below are full spectrum pictures with the  HPF in and out. With the filter in, I can set the LNA to +30dB if I desire. Usually, you want to set the LNA gain to just enough so that you see external antenna noise floor exceed any internal noise floor. This depends on the antenna but most people will need to set their HL2 in the +10 to +12 dB range to meet this and achieve proper sensitivity. Some people like to set the LNA gain higher but you may then just be amplifying both noise and signal, and only introducing more noise due to the greater amplification. With the HPF engaged, I would like to hear if some are not able to set the LNA to a level where external antenna noise is noticed without excessive ADC clipping. Please refer to the LEDs on the HL2 for excessive clipping. Both LEDs on many times a second is bad. Seeing both LEDs on a once or twice in a few seconds is okay.

73,

Steve
kf7o

Without HPF. Notice strongest signals are in the AM band and clipping at +20dB.

40mlpf.png

With HPF. Notice AM signals are no longer the strongest. No clipping even at +20dB RfLna setting.
hpf_40mlpf.png
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