- This topic has 31 replies, 8 voices, and was last updated 15 years ago by
.
-
Posts
-
Hello boys…!!!!!!!
I’m very happy that my idea is reality…..!!!!!!!!…
I think that best configuration….is :PC(BPP)(mpx+airomate)——>optical out—>>>>> Dss Transimitter
Is it possible having a prototipe to make some experiments??????
Thank you….
I’m very happy….!!!
Do you think about this Leif..????Rocco…
Excuse me, but what is the advantage of digital MPX like you guys are talking about in this topic?
[quote author=”sigmacom”]@Boki:
There is no question about this, it will give *THE* perfect results.[/quote]
I was thinking the same but my ears need to hear that! 🙂I asked that because i can’t find unit with digital (Optical) MPX input. Hmm Optical, i am not sure is there any soundcard with 192kHz optical output. Anyway…
It is important to me, and i think to others, because with all our softwares we have complete MPX signal, audio processing, rds, clippers, .. etc…More important is that your unit is not using PLL (x times), but straight to FM band.
again,
Great Job!
^bestI believe that in ideal circumstances, analog mpx will perform and sound just as good as digital mpx.
However, using digital mpx, all of a sudden interference, tilt, frequency response etc etc are all taken out of the equation.
Taking it one step further (as Sigmacom has done), doing direct-to-channel FM synthesis, is even better. That FM carrier WILL be the most accurate representation of the incoming MPX, keeping everything digital as far as humanly possible. I can’t wait to hear this thing, especially not if it’s affordable!
///Leif
I find that the use of two PLL mixing stages to provide the RVR exciter DDS signal up conversion for FM band frequencies to be a bit unusual and an inefficient design approach.
It’s far easier to use class C signal multiplier stages for the up conversion. Frequency doublers (or triplers or quadruplers) is a tried and true simple method to take a low frequency signal sources and make them higher. But not having taken a look at the inner workings of the RVR exciter I can’t validate the PLL up-conversion approach though.There is nothing wrong with PLL designs provided the loop filter is properly engineered. As a designer I have achieved PLL designs with low phase noise in the order of 110-130dBc routinely. Noise this low down is completely inaudible. From a listeners perspective, more phase noise is introduced by the local PLL oscillator mixer of their consumer grade receiver than from the transmitter exciter. Also DDS synthesizers are not immune to noise either. Source clock jitter will provide phase error noise on the DDS baseband output along with lousy post DSS reconstruction filtering.
Applying modulation indices and bandwidths as required to satisfy the broadcast standards to a directly modulated crystal oscillator is not possible. This is because the motional mass of the quartz crystal is too great and cannot provide sufficient deflection to give the needed deviation. Plus the dampening effect of the motional mass acts like a low pass filter whereby the MPX signal will be filtered out. Long ago many TX manufacturers used ovenized crystals as the frequency determining component for their exciters, but had to apply phase modulation to generate the modulation indices for FM broadcast. These crystals were typicaly in the 100’s of kHz’s and fed a whole string of class-C multiplier stages to arrive at required frequency.
GE, Gates and Harris had their own unique way of applying the phase modulation to the multiplier stages. (i.e. Serrasoid to name one)Leif, DDS synthesis does not use a processor running an "algorithm". It’s essentially a clocked state machine that uses a 1/4 sine wave sine look-up table to drive a D/A converter. This converter output through smoothing filters reconstructs the FM signal (much like the sound card makes the MPX signal). By taking a numeric constant (modulation point) and adding it to the internal phase accumulators will generate a phase modulated output signal (FM). Because all the modulation is done in the digital domian, it requires a CPU to prep the audio sample point (plus any MPX signal data) into the proper numeric value prior to sending it on to the DDS chip.
Want to know more about DDS devices visit the Analog Devices Web site. They make lot’s of these kinds of chips.
http://www.analog.com/en/rfif-component … index.htmlThere is an application note that shows how to build a FM transmitter using one of their DSS chips with a companion DSP processor.
Hi Sparky!
About the xtals in my previous post, I didn’t mean to modulate the xtal osc, but to use it instead of VCO+PLL LO.
I don’t know what loop filter they have in their (RVR DDS exciter) PLL LO, but they could avoid all that upconversion chain (DDS 8 MHz –> IF 70 MHz –> Final 87.5-108 MHz)
Either by multiplication or by using an ADI DDS (as you said), or an FPGA with TxDAC, or, or, or…I believe you agree with that! 🙂
Ohhhh my Goooooodnessss!
Sigmacom are you Greek and living to Greece ?
Tell me where to come and find you!
Spanky can you suggest me 2 of the most stable PLL exciters that have a cost less than 2500 dollars ?
( or can someone of you build a 30 watt exciter without stereo and rds encoder inside ) and i will give him 2500 dollars through paypal immediately! ( i use an rvr exciter which has many problems with low frequencies 30, 40 even 50 Hz ! )
I trust you 2 guys cos you write and show your "hardware RF knowledge" !the station i operate : rythm@s 1o4,5 in Herakli0n city
the breakaway fm …modulation power i have achieved at 75 KHz ..needs a new exciter to work stable.. 🙂
Yes, I am from and live in Greece – Thessaloniki. 🙂
After this DDS, I can’t suggest any other exciter about linear frequency response…
[quote author=”Sparky”]ILeif, DDS synthesis does not use a processor running an "algorithm". It’s essentially a clocked state machine that uses a 1/4 sine wave sine look-up table to drive a D/A converter. This converter output through smoothing filters reconstructs the FM signal (much like the sound card makes the MPX signal). By taking a numeric constant (modulation point) and adding it to the internal phase accumulators will generate a phase modulated output signal (FM). Because all the modulation is done in the digital domian, it requires a CPU to prep the audio sample point (plus any MPX signal data) into the proper numeric value prior to sending it on to the DDS chip.
[/quote]Interesting!
This is basically how I wrote the virtual FM exciter in MpxTool (used for the Stokkemasker display). I was lazy though and used a full sine wave look-up table, and then also interpolation of the output values for increased resolution. Mine only runs at 1.536 MHz sampling rate though.
But hey, I was under the impression that DDS meant there’d be no upconversion at all, that the signal was generated at the correct frequency already to begin with.
Or, perhaps that’s what "direct to channel" means?
I’m confused. Could someone clear up the terminology for me? 🙂
///Leif
quote :But hey, I was under the impression that DDS meant there’d be no upconversion at all, that the signal was generated at the correct frequency already to begin with.The answer to this is yes… and no, it can be both. The design of DDS circuits are largely governed by the availability of the silicon that can clock at the appropriate rates for the final frequency of interest. Also factor in tuning resolution, cost, and power consumption will ultimately shape the overall design topology.
[quote author=”Sparky”]
quote :But hey, I was under the impression that DDS meant there’d be no upconversion at all, that the signal was generated at the correct frequency already to begin with.The answer to this is yes… and no, it can be both. The design of DDS circuits are largely governed by the availability of the silicon that can clock at the appropriate rates for the final frequency of interest. Also factor in tuning resolution, cost, and power consumption will ultimately shape the overall design topology.[/quote]
Sparky, thank you, but.. that wasn’t all that clear, really. How about a description of the terminology as you understand it?
For example, what would you call a direct-to-channel exciter to differentiate it from those that use some kind of frequency doubling? For example, I’m pretty sure Nautel’s M50 is completely free from doubling, and if we’re aiming for maximum accuracy and transparency, this would appear to be an important factor.
///Leif
Leif, may I help on that:
Suppose you want to create DDS + FM modulation in 87.5-108MHz band. So, if…a) You have a fast DDS, able to produce 108 MHz sine output, then you have "Direct-to-channel" exciter.
Nautel’s M50, IS "Direct-to-channel", mine IS "Direct-to-channel".b) You don’t have a fast one, you do DDS at a lower frequency and then upconvert it to 87.5-108MHz by using extra analog RF circuitry (VCO+PLL+Mixers+BPF, or multiplication, or whatever).
This is like RVR’s concept, and it is NOT considered as "Direct-to-channel" exciter.Did this helped? 🙂
@ Sigmacom, I would most certainly be interested in trying out a prototype I would be able to use it in a real environment and report the results. What kind of output power would your exciter generate?
[quote author=”yorkie98″]@ Sigmacom, I would most certainly be interested in trying out a prototype I would be able to use it in a real environment and report the results. What kind of output power would your exciter generate?[/quote]
Yes yorkie98, that kind of prototype I have in mind, too! 🙂I have to redesign some things, because the "D-MPX" exciter you guys asked here, is a bit different from what I’ve already designed (*)!
This might take me a few days, so please wait a bit!(*) The first prototype is a 3U 19" rackmounted chassis, with modular design (see some pics of modules I’ve posted earlier).
The "AES/EBU card" can be exchanged with "analog audio L/R card", or 2 Mbit/s (E1) card. The "modulator card" can be exchanged with "DAB modulator card". The 30W RF card goes up to 500 MHz, so it needs replacement only for DAB @ L-Band.
In a few words, it has been designed to offer flexible configuration.
- The forum ‘Breakaway Professional Products – [discontinued]’ is closed to new topics and replies.