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Author Topic: Effectiveness of SDR rsp Filters?  (Read 4085 times)
KC2NLT
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Posts: 92




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« on: September 19, 2017, 07:34:14 AM »

Those who are familiar with the rsp 1 and 2 products, how would you compare the built in bandpass, notch filters and IF to a solid state receiver with crystal filters?

Are they as effective at reducing intermodulation or interference from adjacent frequencies or bands?

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KE2TR
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Posts: 616




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« Reply #1 on: September 19, 2017, 12:35:02 PM »

Just look at there band pass filter system on there block diagram and what you will see is an HF block, VHF block and so on as you go up in frequency, they have no real out of band protection like what you have in modern HF rigs today. They are still best served if you have then hooked thru an IF out port so at least you are using the band pass filters in the front end in most radio's but were you will ahve trouble is with using them with an 7300 were you just modify the antenna input before the band pass filters, it would work far better used after them. Otherwise you have an open HF front ended on that product at HF.
I would love to see other companies beside Flex who in there top of there line 6600 will have amateur band band pass filters in there front end to give superior amateur band performance not the BS general coverage crap we have in way too many radio's today, Why is it soo hard for the big three from Japan to figure this part of the equation out, most hams give a choice would rather have stout amateur band bandpass filters in there radio's over this general coverage crap but they keep on shoving this crap in the ham's face but Flex has got the right idea. Who knows maybe the rest will catch up/on.
BTW Elad got it in there duo rig but comes at a price, you just don't get something for nothing.
« Last Edit: September 19, 2017, 12:45:42 PM by KE2TR » Logged
N2DTS
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Posts: 732




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« Reply #2 on: September 19, 2017, 12:55:56 PM »

Lets not mix up band pass filters with IF filters or filters at the passband level.

With direct digital sampling, band pass filters can help a lot with strong local signals and big antenna's on poor (12 bit or 14 bit) low end sdr radios.
That is not the same as the filter you use to separate one cw signal from another 100 Hz away. SDR radios do a very good job of that, plus the filter shape and ripple is MUCH better then any analog filter.

I think, if you could buy an sdr with a 20 bit A/D converter you could get a great dynamic range and would not need ANY band pass filters at all.
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AC7CW
Member

Posts: 969




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« Reply #3 on: September 19, 2017, 08:50:59 PM »

Just look at there band pass filter system on there block diagram and what you will see is an HF block, VHF block and so on as you go up in frequency, they have no real out of band protection like what you have in modern HF rigs today. They are still best served if you have then hooked thru an IF out port so at least you are using the band pass filters in the front end in most radio's but were you will ahve trouble is with using them with an 7300 were you just modify the antenna input before the band pass filters, it would work far better used after them. Otherwise you have an open HF front ended on that product at HF.
I would love to see other companies beside Flex who in there top of there line 6600 will have amateur band band pass filters in there front end to give superior amateur band performance not the BS general coverage crap we have in way too many radio's today, Why is it soo hard for the big three from Japan to figure this part of the equation out, most hams give a choice would rather have stout amateur band bandpass filters in there radio's over this general coverage crap but they keep on shoving this crap in the ham's face but Flex has got the right idea. Who knows maybe the rest will catch up/on.
BTW Elad got it in there duo rig but comes at a price, you just don't get something for nothing.

I liked that about my IC-730. I hate feeling vulnerable to out of band strong signals. Another way to achieve the filtering is to use a high Q mag loop antenna.
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Novice 1958, 20WPM Extra now... (and get off my lawn)
KC2NLT
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Posts: 92




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« Reply #4 on: September 19, 2017, 11:23:53 PM »

Thanks for all the responses, but it seems that the focus is on HF.

How would you say they fair in the VHF and UHF bands?

Thanks.

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GM1FLQ
Member

Posts: 794




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« Reply #5 on: September 20, 2017, 03:50:05 AM »


I would love to see other companies beside Flex who in there top of there line 6600 will have amateur band band pass filters in there front end to give superior amateur band performance not the BS general coverage crap we have in way too many radio's today, Why is it soo hard for the big three from Japan to figure this part of the equation out.........


And you think for one minute that they haven't............it's because they are commercially astute, they know they don't need to bother, all they need do is stick a fancy kaleidoscope fishfinder screen on and the sheeples will be queuing up to buy their radios all day long  Wink..........you are now reaping the wonders of the market these "clever" sheeples have created. Not so clever now eh........

      

Geez, and they wonder why the negativity about them  Roll Eyes...........
« Last Edit: September 20, 2017, 04:11:13 AM by GM1FLQ » Logged
N0YXB
Member

Posts: 1121




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« Reply #6 on: September 20, 2017, 08:35:15 AM »

Thanks for all the responses, but it seems that the focus is on HF.

How would you say they fair in the VHF and UHF bands?


On the SDRplay Facebook page there are a lot of posts about ADS-B and weather satellite reception, and other types of VHF and above reception.

Regarding filters, an online brochure says:
Front End Filtering  (Ports A and B) 
Low Pass:  12MHz

Band Pass:  12 – 30MHz, 30 – 60MHz, 60 – 120MHz, 120 – 250MHz, 250 – 300MHz, 300 – 380MHz, 380 – 420MHz, 420 – 1000MHz. 
 
High Pass:  1000MHz 
 
Notch Filters:  FM Filter >60dB 80 – 100MHz  MW Filter >30dB 680 – 1550 kHz
 
Front End Filtering (High Z port)
Low Pass   30MHz
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N2DTS
Member

Posts: 732




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« Reply #7 on: September 20, 2017, 06:32:17 PM »

The rsp2, even with the band pass filters needs very careful gain control.
It can work great but often has all kinds of spurs and trash on it if the gain is not set right.
vhf/uhf seems better but HF on a full size antenna needs manual gain control of both the IF and RF gain controls.
The better sdr's do not (Flex, Anan).
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N0YXB
Member

Posts: 1121




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« Reply #8 on: September 20, 2017, 08:24:57 PM »

The rsp2, even with the band pass filters needs very careful gain control.
It can work great but often has all kinds of spurs and trash on it if the gain is not set right.
vhf/uhf seems better but HF on a full size antenna needs manual gain control of both the IF and RF gain controls.


I enjoy using mine, but that's my experience as well.
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GM1FLQ
Member

Posts: 794




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« Reply #9 on: September 21, 2017, 02:15:13 AM »

The rsp2, even with the band pass filters needs very careful gain control.
It can work great but often has all kinds of spurs and trash on it if the gain is not set right.
vhf/uhf seems better but HF on a full size antenna needs manual gain control of both the IF and RF gain controls.

The better sdr's ..........

........& just to remind - at 10-20 times the price or more  Wink...........

............do not (Flex, Anan).

.......& would kinda hope so........
« Last Edit: September 21, 2017, 02:21:03 AM by GM1FLQ » Logged
N2DTS
Member

Posts: 732




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« Reply #10 on: September 21, 2017, 12:38:58 PM »

Its quite amazing for the price, and often works just as well as a much more expensive unit, but its NOT set and forget.
I have full size fan dipoles up at about 45 feet and the IF and RF gain have to be almost all the way down, otherwise all sorts of trash and noise comes in. That is the limitation of a 12 bit radio with plenty of gain before the A/D converter.



I enjoy using mine, but that's my experience as well.
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W8KFJ
Member

Posts: 55




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« Reply #11 on: September 23, 2017, 12:53:55 PM »

Just thought I would add my experience to what is a very relevant topic.  If you go way back to the tube days, most receivers had pre-selectors.  That was an extra tuning step though, and soon receivers started to sprout band-pass filters for the ham bands.  This worked well unless you wanted to get rid of a 40 meter CW station, while you were on 40 SSB.  Nevertheless, these filters became the norm for the later years of the superhet era.  If you wanted to listen to anything but the ham bands though, you had to either add more filters, or bypass the existing ones.

Enter the SDR era, and confusion abounds.  Not all SDRs are equal.  The direct-CONVERSION type (Flex pre-6000 series, and many others) are only somewhat better than superhets.  The more conversion stages, the worse the intermod products, and the triple-conversion superhet was the worst.  I think it was Yaesu that touted its newer double-conversion radio over its older triple-conversion superhet.  The problem was that ONE conversion stage still existed in even the best SDRs of the day.  Can we get rid of that last remaining conversion stage?

The answer is a resounding YES!  In 2012, Flex-Radio introduced the 6000 (direct-sampling) series with the 6500 and 6700.  Also in 2012, but with less hoopla, Apache Labs in India, introduced the direct-sampling Anan 10, based on the open-source Hermes board developed by the Open HPSDR group, and manufactured by Apache Labs.  These radios didn't just digitize a couple-hundred KILOhertz of the spectrum by using a VFO to down-convert that narrow spectrum, they digitized the whole thing!  By sampling above 60 MEGAhertz, they could digitize everything through ten meters.  Double that to 120 MEGAhertz, and six meters was included.  As Flex touted, this was indeed a game-changer!  With good design, intermod products were practically eliminated.  Radios could now be made WITHOUT pre-selector filters for all but the most demanding situations.

I had already purchased a Flex 3000, but I became intrigued with this direct-sampling stuff, and by December, 2013, I bought a Hermes board.  This was a complete receiver, and half-watt transmitter.  There was no enclosure, and certainly no front-end band-pass filters.  When it arrived, my excitement prevailed.  I loaded the HPSDR software, connected everything and threw the big switch.  Light lit, there was no smoke, and the receiver worked VERY well.  But what could I do to test the improved intermod situation promised by direct-sampling?

I remembered quite a few intermod products appearing in the lower part of the AM broadcast band on the Flex 3k.  They were not there in the Hermes!  Let's try WWVB on 60 KHz.  The Flex 3k had so much intermod from AM broadcast, it was useless.  The Hermes was perfect, as is my Elad FDM-DUO.  I live in the greater Cleveland area, so there are many STRONG AM stations.  A friend, who lives in the boonies, tried his Flex 3k on 60 Khz, and it was fine.  Bottom line – filters are only needed in the most extreme cases!

The good news – prices of the high-speed A/Ds and FPGAs needed for direct-sampling are plummeting!  The ColibriNANO, Elad S1, and KiwiSDR direct-sampling receivers can be had for around $300.  The KiwiSDR is particularly interesting, as it includes a Beaglebone computer and GPS oscillator steering for $300.  It is designed to be used over the internet by up to four simultaneous users.  Although these receivers are14 bit, direct-sampling allows over-sampling techniques to obtain bit depths of 24 – or better.

I will never buy anything but a direct-sampling radio!

Dave   W8KFJ

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K0UA
Member

Posts: 1368




Ignore
« Reply #12 on: September 23, 2017, 01:21:58 PM »

Just thought I would add my experience to what is a very relevant topic.  If you go way back to the tube days, most receivers had pre-selectors.  That was an extra tuning step though, and soon receivers started to sprout band-pass filters for the ham bands.  This worked well unless you wanted to get rid of a 40 meter CW station, while you were on 40 SSB.  Nevertheless, these filters became the norm for the later years of the superhet era.  If you wanted to listen to anything but the ham bands though, you had to either add more filters, or bypass the existing ones.

Enter the SDR era, and confusion abounds.  Not all SDRs are equal.  The direct-CONVERSION type (Flex pre-6000 series, and many others) are only somewhat better than superhets.  The more conversion stages, the worse the intermod products, and the triple-conversion superhet was the worst.  I think it was Yaesu that touted its newer double-conversion radio over its older triple-conversion superhet.  The problem was that ONE conversion stage still existed in even the best SDRs of the day.  Can we get rid of that last remaining conversion stage?

The answer is a resounding YES!  In 2012, Flex-Radio introduced the 6000 (direct-sampling) series with the 6500 and 6700.  Also in 2012, but with less hoopla, Apache Labs in India, introduced the direct-sampling Anan 10, based on the open-source Hermes board developed by the Open HPSDR group, and manufactured by Apache Labs.  These radios didn't just digitize a couple-hundred KILOhertz of the spectrum by using a VFO to down-convert that narrow spectrum, they digitized the whole thing!  By sampling above 60 MEGAhertz, they could digitize everything through ten meters.  Double that to 120 MEGAhertz, and six meters was included.  As Flex touted, this was indeed a game-changer!  With good design, intermod products were practically eliminated.  Radios could now be made WITHOUT pre-selector filters for all but the most demanding situations.

I had already purchased a Flex 3000, but I became intrigued with this direct-sampling stuff, and by December, 2013, I bought a Hermes board.  This was a complete receiver, and half-watt transmitter.  There was no enclosure, and certainly no front-end band-pass filters.  When it arrived, my excitement prevailed.  I loaded the HPSDR software, connected everything and threw the big switch.  Light lit, there was no smoke, and the receiver worked VERY well.  But what could I do to test the improved intermod situation promised by direct-sampling?

I remembered quite a few intermod products appearing in the lower part of the AM broadcast band on the Flex 3k.  They were not there in the Hermes!  Let's try WWVB on 60 KHz.  The Flex 3k had so much intermod from AM broadcast, it was useless.  The Hermes was perfect, as is my Elad FDM-DUO.  I live in the greater Cleveland area, so there are many STRONG AM stations.  A friend, who lives in the boonies, tried his Flex 3k on 60 Khz, and it was fine.  Bottom line – filters are only needed in the most extreme cases!

The good news – prices of the high-speed A/Ds and FPGAs needed for direct-sampling are plummeting!  The ColibriNANO, Elad S1, and KiwiSDR direct-sampling receivers can be had for around $300.  The KiwiSDR is particularly interesting, as it includes a Beaglebone computer and GPS oscillator steering for $300.  It is designed to be used over the internet by up to four simultaneous users.  Although these receivers are14 bit, direct-sampling allows over-sampling techniques to obtain bit depths of 24 – or better.

I will never buy anything but a direct-sampling radio!

Dave   W8KFJ



Dave, thank you for that comprehensive, yet easy to understand synopsis on SDR.   73 James K0UA
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ZENKI
Member

Posts: 1424




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« Reply #13 on: September 23, 2017, 03:43:30 PM »

Rather than thinking of direct sampling receivers as just modern versions of  just another modern technology shortwave receiver, we should start thinking of them as measuring receivers than can be a wonderful test platform. The Red Pattaya is giving us a glimpse of what these wonderful direct sampling receivers can  be as  a test instrument platform.

Direct sampling receivers can make excellent:

Direction finding systems
Behave as accurate wattmeters
Plot antenna polar patterns
Make excellent EMC analyzers with the appropriate CISPR bandwidth filters and Quasi Peak level  indication.
Make very good beam steering  noise cancelling systems
Make  great spectrum analyzers

Hams should encourage manufacturers to drive these  direct sampling receivers into these directions and give ham tools that was not possible on legacy analog shortwave receivers. Hams have not even begun to scratch the surface  of the future potential of direct sampling receivers.... theres much more to come of we  open our minds and educate ourselves to the possibilities.
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N2DTS
Member

Posts: 732




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« Reply #14 on: September 23, 2017, 05:21:54 PM »

When you get to direct sampling and 16 bits, it gets VERY good.
14 bits is almost as good (some Anan's), 12 bits is not so great.

But within the dynamic range of the 12 bit radios, the performance is very good.

You also have to consider the software used and the phase noise of the clock chips, along with preamps and attenuators.
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